Fix potential infinite loop when iterating over redeclarations of an ObjMethodDecl, resulting from invalid code.

Check for invalid decls in ObjCMethodDecl::getNextRedeclaration(); otherwise if we start from an invalid redeclaration
of an @implementation we would move to the @interface and not reach the original declaration again.

Fixes rdar://14024851

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182951 91177308-0d34-0410-b5e6-96231b3b80d8