[clang] [Clang] Improve EmitClangAttrSpellingListIndex (PR #114899)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lldb-x86_64-debian` running on `lldb-x86_64-debian` while building `clang` at step 4 "build".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/162/builds/10003

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

```
Step 4 (build) failure: build (failure)
0.964 [1257/3/1] Generating VCSRevision.h
2.593 [1254/5/2] Building CXX object tools/llvm-config/CMakeFiles/llvm-config.dir/llvm-config.cpp.o
2.659 [1253/5/3] Linking CXX executable bin/llvm-config
2.845 [1253/4/4] Building CXX object tools/clang/utils/TableGen/CMakeFiles/clang-tblgen.dir/ClangAttrEmitter.cpp.o
FAILED: tools/clang/utils/TableGen/CMakeFiles/clang-tblgen.dir/ClangAttrEmitter.cpp.o 
/usr/bin/clang++ -DCLANG_EXPORTS -DGTEST_HAS_RTTI=0 -DLLVM_BUILD_STATIC -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/worker/2.0.1/lldb-x86_64-debian/build/tools/clang/utils/TableGen -I/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/clang/utils/TableGen -I/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/clang/include -I/home/worker/2.0.1/lldb-x86_64-debian/build/tools/clang/include -I/home/worker/2.0.1/lldb-x86_64-debian/build/include -I/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -Wno-nested-anon-types -O3 -DNDEBUG  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -std=c++17 -MD -MT tools/clang/utils/TableGen/CMakeFiles/clang-tblgen.dir/ClangAttrEmitter.cpp.o -MF tools/clang/utils/TableGen/CMakeFiles/clang-tblgen.dir/ClangAttrEmitter.cpp.o.d -o tools/clang/utils/TableGen/CMakeFiles/clang-tblgen.dir/ClangAttrEmitter.cpp.o -c /home/worker/2.0.1/lldb-x86_64-debian/llvm-project/clang/utils/TableGen/ClangAttrEmitter.cpp
/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/clang/utils/TableGen/ClangAttrEmitter.cpp:3869:51: error: reference to local binding 'FS' declared in enclosing function 'clang::EmitClangAttrSpellingListIndex'
            [&](StringRef N) { return N.size() == FS.name().size(); });
                                                  ^
/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/clang/utils/TableGen/ClangAttrEmitter.cpp:3863:28: note: 'FS' declared here
    for (const auto &[Idx, FS] : enumerate(Spellings)) {
                           ^
1 error generated.
3.621 [1253/3/5] Building CXX object lib/Object/CMakeFiles/LLVMObject.dir/IRSymtab.cpp.o
10.819 [1253/2/6] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
12.032 [1253/1/7] Building CXX object lib/LTO/CMakeFiles/LLVMLTO.dir/LTO.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/114899