[llvm-ar] Add more tests for errors in opening archives

Add 2 more tests for the function performOperation under llvm-ar.cpp. It
tests the scenario when the archive could not be opened for reasons
other than `no_such_file_or_directory` In particular, it tests for the
cases `permission_denied` and `is_a_directory` for the target archive.

Reviewed By: alexshap, jhenderson, smeenai, MaskRay

Differential Revision: https://reviews.llvm.org/D80838