[llvm] Silence warning when compiling with MSVC targetting x86

This fixes:
```
[1304/6998] Building CXX object lib\ObjectYAML\CMakeFiles\LLVMObjectYAML.dir\COFFYAML.cpp.obj
C:\git\llvm-project\llvm\lib\ObjectYAML\COFFYAML.cpp(561): warning C4018: '<': signed/unsigned mismatch
```