[ObjC] Prevent infinite loops when iterating over redeclaration

of a method that was declared in an invalid interface

This commit fixes an infinite loop that occurs when clang tries to iterate over
redeclaration of a method that was declared in an invalid @interface. The
existing validity checks don't catch this as that @interface is a duplicate of
a previously declared valid @interface declaration, so we have to verify that
the found redeclaration is in a valid declaration context.

rdar://29220965

Differential Revision: https://reviews.llvm.org/D26664

llvm-svn: 287530