[clang] Silence warning when building with MSVC targetting x86

This fixes:
```
[3786/6996] Building CXX object tools\clang\lib\AST\CMakeFiles\obj.clangAST.dir\AttrDocTable.cpp.obj
C:\git\llvm-project\clang\lib\AST\AttrDocTable.cpp(24): warning C4018: '<': signed/unsigned mismatch
```