COFF: Implement /pdbaltpath flag.

I needed to revert r330223 because we were embedding an absolute PDB
path in the .rdata section, which ended up being laid out before the
.idata section and affecting its RVAs. This flag will let us control
the embedded path.

Differential Revision: https://reviews.llvm.org/D45747

llvm-svn: 330232