[AST] Fix crashes caused by redeclarations in hidden prototypes

ObjCContainerDecl.getMethod returns a nullptr by default when the
container is a hidden prototype. Callsites where the method is being
looked up on the redeclaration's own container should skip this check
since they (rightly) expect a valid method to be found.

Resolves rdar://69444243

Reviewed By: akyrtzi

Differential Revision: https://reviews.llvm.org/D89024