[llvm] [ADT] Teach StringRef to derive from std::string_view (PR #113752)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-linux-test-suite` running on `ppc64le-clang-test-suite` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/95/builds/5461

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-unified-tree) failure: build (failure)
...
3.254 [5170/209/22] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/InstructionCost.cpp.o
3.322 [5170/208/23] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/CSKYAttributes.cpp.o
3.333 [5170/207/24] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/circular_raw_ostream.cpp.o
3.336 [5170/206/25] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/SystemUtils.cpp.o
3.403 [5170/205/26] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/raw_os_ostream.cpp.o
3.454 [5170/204/27] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/MemoryBufferRef.cpp.o
3.506 [5170/203/28] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Allocator.cpp.o
3.510 [5170/202/29] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/DJB.cpp.o
3.562 [5170/201/30] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/OptionStrCmp.cpp.o
3.577 [5170/200/31] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/CommandLine.cpp.o
FAILED: lib/Support/CMakeFiles/LLVMSupport.dir/CommandLine.cpp.o 
ccache /usr/lib64/ccache/c++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/build/lib/Support -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/lib/Support -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/build/include -I/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wno-comment -Wno-misleading-indentation -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17 -UNDEBUG  -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Support/CMakeFiles/LLVMSupport.dir/CommandLine.cpp.o -MF lib/Support/CMakeFiles/LLVMSupport.dir/CommandLine.cpp.o.d -o lib/Support/CMakeFiles/LLVMSupport.dir/CommandLine.cpp.o -c /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/lib/Support/CommandLine.cpp
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/lib/Support/CommandLine.cpp: In function ‘void ExpandBasePaths(llvm::StringRef, llvm::StringSaver&, const char*&)’:
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/lib/Support/CommandLine.cpp:1127:60: error: call of overloaded ‘StringRef(const llvm::StringLiteral&)’ is ambiguous
   for (StringRef::size_type TokenPos = ArgString.find(Token);
                                                            ^
In file included from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringMapEntry.h:19,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringMap.h:17,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/Support/CommandLine.h:26,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/lib/Support/CommandLine.cpp:18:
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringRef.h:103:28: note: candidate: ‘constexpr llvm::StringRef::StringRef(std::string_view)’
     /*implicit*/ constexpr StringRef(std::string_view Str) : Base(Str) {}
                            ^~~~~~~~~
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringRef.h:51:26: note: candidate: ‘constexpr llvm::StringRef::StringRef(const llvm::StringRef&)’
   class LLVM_GSL_POINTER StringRef : public std::string_view {
                          ^~~~~~~~~
In file included from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringMapEntry.h:19,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringMap.h:17,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/Support/CommandLine.h:26,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/lib/Support/CommandLine.cpp:18:
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringRef.h:313:26: note:   initializing argument 1 of ‘size_t llvm::StringRef::find(llvm::StringRef, size_t) const’
     [[nodiscard]] size_t find(StringRef Str, size_t From = 0) const;
                          ^~~~
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/lib/Support/CommandLine.cpp:1129:49: error: call of overloaded ‘StringRef(const llvm::StringLiteral&)’ is ambiguous
        TokenPos = ArgString.find(Token, StartPos)) {
                                                 ^
In file included from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringMapEntry.h:19,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringMap.h:17,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/Support/CommandLine.h:26,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/lib/Support/CommandLine.cpp:18:
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringRef.h:103:28: note: candidate: ‘constexpr llvm::StringRef::StringRef(std::string_view)’
     /*implicit*/ constexpr StringRef(std::string_view Str) : Base(Str) {}
                            ^~~~~~~~~
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringRef.h:51:26: note: candidate: ‘constexpr llvm::StringRef::StringRef(const llvm::StringRef&)’
   class LLVM_GSL_POINTER StringRef : public std::string_view {
                          ^~~~~~~~~
In file included from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringMapEntry.h:19,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/ADT/StringMap.h:17,
                 from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/include/llvm/Support/CommandLine.h:26,

```

</details>

https://github.com/llvm/llvm-project/pull/113752