[clang] Silence warning when compiling with MSVC targetting x86

This fixes:
```
[3963/6996] Building CXX object tools\clang\lib\CodeGen\CMakeFiles\obj.clangCodeGen.dir\CGExpr.cpp.obj
C:\git\llvm-project\clang\lib\CodeGen\CGExpr.cpp(3808): warning C4018: '<=': signed/unsigned mismatch
```