[clang] [Clang] Optimize some `getBeginLoc` implementations (PR #141058)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-armv8-quick` running on `linaro-clang-armv8-quick` while building `clang` at step 4 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/154/builds/16766

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

```
Step 4 (build stage 1) failure: 'ninja' (failure)
...
[63/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/DeclFriend.cpp.o
[64/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/DeclGroup.cpp.o
[65/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/DeclObjC.cpp.o
[66/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/DeclOpenACC.cpp.o
[67/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/DeclOpenMP.cpp.o
[68/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/DeclPrinter.cpp.o
[69/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/DeclTemplate.cpp.o
[70/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/DynamicRecursiveASTVisitor.cpp.o
[71/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/ParentMapContext.cpp.o
[72/1440] Building CXX object tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/Expr.cpp.o
FAILED: tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/Expr.cpp.o 
/usr/local/bin/c++ -DCLANG_EXPORTS -DGTEST_HAS_RTTI=0 -D_DEBUG -D_FILE_OFFSET_BITS=64 -D_GLIBCXX_ASSERTIONS -D_GNU_SOURCE -D_LARGEFILE_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/tcwg-buildbot/worker/clang-armv8-quick/stage1/tools/clang/lib/AST -I/home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang/lib/AST -I/home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang/include -I/home/tcwg-buildbot/worker/clang-armv8-quick/stage1/tools/clang/include -I/home/tcwg-buildbot/worker/clang-armv8-quick/stage1/include -I/home/tcwg-buildbot/worker/clang-armv8-quick/llvm/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 -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/Expr.cpp.o -MF tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/Expr.cpp.o.d -o tools/clang/lib/AST/CMakeFiles/obj.clangAST.dir/Expr.cpp.o -c /home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang/lib/AST/Expr.cpp
../llvm/clang/lib/AST/Expr.cpp:1473:46: error: static assertion failed due to requirement 'sizeof(clang::CXXOperatorCallExpr) == 32': we assume CXXOperatorCallExpr is 32 bytes
 1473 | static_assert(sizeof(SourceLocation) == 4 && sizeof(CXXOperatorCallExpr) == 32,
      |                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../llvm/clang/lib/AST/Expr.cpp:1473:74: note: expression evaluates to '28 == 32'
 1473 | static_assert(sizeof(SourceLocation) == 4 && sizeof(CXXOperatorCallExpr) == 32,
      |                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
1 error generated.
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/141058