[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