[lld-macho] Group undefined symbol diagnostics by symbol

ld64.lld used to print the "undefined symbol" line for each reference to
an undefined symbol previously:

  ld64.lld: error: undefined symbol: _foo
  >>> referenced by /path/to/bar.o:(symbol _baz+0x0)

  ld64.lld: error: undefined symbol: _foo
  >>> referenced by /path/to/bar.o:(symbol _quux+0x1)

Now they are deduplicated:

  ld64.lld: error: undefined symbol: _foo
  >>> referenced by /path/to/bar.o:(symbol _baz+0x0)
  >>> referenced by /path/to/bar.o:(symbol _quux+0x1)

As with the other lld ports, only the first 3 references are printed.

Differential Revision: https://reviews.llvm.org/D127753