[clang] On Windows, silence warning in `CFGBackEdgesTest` with MSVC (#187939)
This fixes:
```
[5804/7029] Building CXX object tools\clang\unittests\CMakeFiles\AllClangUnitTests.dir\Analysis\CFGBackEdgesTest.cpp.obj
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\include\xutility(721): warning C4018: '>': signed/unsigned mismatch
...
C:\git\llvm-project\clang\unittests\Analysis\CFGBackEdgesTest.cpp(170): note: see the first reference to 'testing::internal::PredicateFormatterFromMatcher<testing::internal::SizeIsMatcher<testing::internal::GtMatcher<int>>>::operator ()' in 'clang::analysis::`anonymous-namespace'::CFGBackEdgesTest_WhileWithContinueLoop_Test::TestBody'
```