[lld-macho] Use source information in duplicate symbol errors

Similarly to how undefined symbol diagnostics were changed in D128184,
we now show where in the source file duplicate symbols are defined at:

  ld64.lld: error: duplicate symbol: _foo
  >> defined in bar.c:42
  >>            /path/to/bar.o
  >> defined in baz.c:1
  >>            /path/to/libbaz.a(baz.o)

For objects that don't contain DWARF data, the format is unchanged.

A slight difference to undefined symbol diagnostics is that we don't
print the name of the symbol on the third line, as it's already
contained on the first line.

Differential Revision: https://reviews.llvm.org/D128425