Only computeRelativePath() on new members

Summary:
When using thin archives, and processing the same archive multiple times, we were mangling existing entries.  The root cause is that we were calling computeRelativePath() more than once.   Here, we only call it when adding new members to an archive.

Note that D27218 changes the way thin archives are printed, and will break the new unit test included here.  Depending on which one lands first, the other will need to be slightly modified.

Reviewers: rafael, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27217

llvm-svn: 288280