Revert "Speed-up ObjCMethodDecl::getOverriddenMethods()."

This reverts commit r179436.

Due to caching, it was possible that we could miss overridden methods that
were introduced by categories later on.

Along with reverting the commit I also included a test case that would have caught this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179547 91177308-0d34-0410-b5e6-96231b3b80d8