[objc_direct] fix codegen for mismatched Decl/Impl return types

For non direct methods, the codegen uses the type of the Implementation.
Because Objective-C rules allow some differences between the Declaration
and Implementation return types, when the Implementation is in this
translation unit, the type of the Implementation should be preferred to
emit the Function over the Declaration.

Radar-Id: rdar://problem/58797748


Signed-off-by: Pierre Habouzit <phabouzit@apple.com>
Differential Revision: https://reviews.llvm.org/D73208