Handle archives with paths in the names.

Handle archives with paths in the names.

We always create archives with just he filename as the member name, but
other archives can put a more complicated path in there.

This patches handles it by computing just the filename as we do when
adding a new member.

If storing the path is important for some reason, we should probably
have an orthogonal option for doing that and do it for both old and new
members.

Fixes pr25877.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256001 91177308-0d34-0410-b5e6-96231b3b80d8