Fix <rdar://problem/6315646> clang on xcode: error: invalid operands to binary...

Fix <rdar://problem/6315646> clang on xcode: error: invalid operands to binary expression ('id<NSTableViewDelegate>' and 'XCExtendedArrayController *').

There is still a bug here (as the FIXME in the test case indicates). Prior to this patch, the bug would generate an error. Now, we simply do nothing (which is less harmful until we can get it right). The complete bug fix will require changing ASTContext::mergeTypes(), which I'd like to defer for now.

llvm-svn: 58241