Reland "[lld/mac] Port typo correction for undefined symbols from ELF port"

The only difference in the reland is that the loop at the top of
getAlternativeSpelling() now calls dyn_cast_or_null() instead
of dyn_cast() -- a file's symbols list can contain null entries.

The test for this might be slightly involved, so I'll land it in
a follow-up, to make the reland similar to the original commit.

Originally reviewed at:
Differential Revision: https://reviews.llvm.org/D135038

This reverts commit 317b5582.