[lld-macho] Print the name of functions containing undefined references

The error used to look like this:

  ld64.lld: error: undefined symbol: _foo
  >>> referenced by /path/to/bar.o

Now it displays the name of the function that contains the undefined
reference as well:

  ld64.lld: error: undefined symbol: _foo
  >>> referenced by /path/to/bar.o:(symbol _baz+0x4)

Differential Revision: https://reviews.llvm.org/D127696