[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