[ObjC] Add a command line flag that disables recognition of objc_direct for testability

Programmers would like to be able to test direct methods by calling them from a
different linkage unit or mocking them, both of which are impossible. This
patch adds a flag that effectively disables the attribute, which will fix this
when enabled in testable builds. rdar://71190891

Differential revision: https://reviews.llvm.org/D95845