[lld-macho] Support re-exports of individual symbols

Specifically, we support this:

  ld64.lld -dylib foo.o libbar.dylib -exported_symbol _bar -o libfoo.dylib

Where `_bar` is defined in libbar.dylib.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D144153