[llvm] Silence warning when compiling with MSVC targetting x86

This fixes:
```
[1265/6998] Building CXX object lib\Object\CMakeFiles\LLVMObject.dir\DXContainer.cpp.obj
C:\git\llvm-project\llvm\lib\Object\DXContainer.cpp(344): warning C4018: '<': signed/unsigned mismatch
```