[PECOFF] Ignore /pdbaltpath option.

/PDBALTPATH:<path> is an option to embed a different path for the PDB file to
the binary than the actual PDB file location. Because we don't support PDB
file, we'll just ignore the option for now.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@191273 91177308-0d34-0410-b5e6-96231b3b80d8