[lld/mac] Don't emit stabs entries for functions folded during ICF

This matches ld64, and makes dsymutil work better with lld's output.
Fixes PR54783, see there for details.

Reduces time needed to run dsymutil on Chromium Framework from 8m30s
(which is already down from 26 min with D123218) to 6m30s and removes
many lines of "could not find object file symbol for symbol" from dsymutil output
(previously: several MB of those messages, now dsymutil is completely silent).

Differential Revision: https://reviews.llvm.org/D123252