Sema: Simplify the check if a method returns an instance of the class.

Just checking if the parent of the method is the same as the return type
should be sufficient. Also fixes PR17587.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192802 91177308-0d34-0410-b5e6-96231b3b80d8
Update comment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192804 91177308-0d34-0410-b5e6-96231b3b80d8