[lld-macho] Do not error out on dead stripped duplicate symbols

Builds that error out on duplicate symbols can still succeed if the symbols
will be dead stripped. Currently, this is the current behavior in ld64.
https://github.com/apple-oss-distributions/ld64/blob/main/src/ld/Resolver.cpp#L2018.
In order to provide an easier to path for adoption, introduce a new flag that will
retain compatibility with ld64's behavior (similar to `--deduplicate-literals`). This is
turned off by default since we do not encourage this behavior in the linker.

Reviewed By: #lld-macho, thakis, int3

Differential Revision: https://reviews.llvm.org/D134794