[clang] On Windows, silence warning when building with MSVC (#187937)
This fixes:
```
[2124/7029] Building CXX object tools\clang\lib\AST\CMakeFiles\obj.clangAST.dir\TemplateBase.cpp.obj
C:\git\llvm-project\clang\lib\AST\TemplateBase.cpp(753): warning C4312: 'reinterpret_cast': conversion from 'clang::SourceLocation::UIntTy' to 'clang::TemplateArgumentLocInfo::LocOrPointer' of greater size
```