[lld-macho] Handle non-extern symbols marked as private extern

Previously, we asserted that such a case was invalid, but in fact
`ld -r` can emit such symbols if the input contained a (true) private
extern, or if it contained a symbol started with "L".

Non-extern symbols marked as private extern are essentially equivalent
to regular TU-scoped symbols, so no new functionality is needed.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D104502