[lld-macho] Prevent assertions for aliases to weak_def_can_be_hidden symbols

In https://reviews.llvm.org/D137982 we found that on Mach-O private
aliases could trigger an assert in lld when the aliasee was a
weak_def_can_be_hidden symbol.

This appears to be incorrect, and should be allowed in Mach-O.
Disallowing this behavior is also inconsistent with how ld64 handles
a private alias to weak_def_can_be_hidden symbols.

This patch removes the assert and tests that LLD handles such aliases
gracefully.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D141082