[llvm-ar] Fix when llvm-ar fails to replace existing members when updating a thin archive

As seen in https://github.com/llvm/llvm-project/issues/55023 when a thin
archive is updated when not in the CWD, replacement does not work as
expected. This change fixes the relative file path comparison so the
correct files are updated.

Differential Revision: https://reviews.llvm.org/D138218