[lld-macho] Ignore -debug_variant flag

As far as I can tell all this flag does in ld64 is warn about cases
where you're building dylibs that are going to be part of the shared
cache, which I assume only apple is doing. Strangely I have seen this
flag being passed in the wild so this silences the warning about it.

Differential Revision: https://reviews.llvm.org/D145200