[llvm-ar] Make llvm-lib behave more like the MSVC archiver

Summary:
Use the filepath used to open the archive member as the archive member
name instead of the file basename. This path might be absolute or
relative.  This is important because the archive member name will show
up in the PDB, and we want our PDBs to look as much like MSVC's as
possible.

This also helps avoid an issue in our PDB module descriptor writing
code, which assumes that all module names are unique. Relative paths
still aren't guaranteed to be unique, but they're much better than
basenames, which definitely aren't unique.

Reviewers: ruiu, zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33575

llvm-svn: 305223