[LLD] Silence warning when compiling with MSVC targetting x86

This fixes:
```
[5383/6995] Building CXX object tools\lld\COFF\CMakeFiles\lldCOFF.dir\PDB.cpp.obj
C:\git\llvm-project\lld\COFF\PDB.cpp(835): warning C4018: '>=': signed/unsigned mismatch
```