It updates two error messages under `performOperation` in the file
llvm-ar.cpp. Furthermore, it also updates tests that print out these
error messages: `llvm/test/Object/ar-create.test` and
`llvm/test/tools/llvm-ar/print.test`.
Reviewed By: jhenderson, MaskRay
Differential Revision: https://reviews.llvm.org/D80846
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