[lld-macho] Don't emit spurious dupe method warnings for +load methods

+load methods are static initializers and treated specially by the
runtime: all +load methods for a class & its categories are called when
loading that class, unlike regular methods where only one definition
will get called per message. Thus, there is no need to check for
duplicates.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D147230