Downgrade incompatibilities with objc qualified types (e.g. id <P>) to warnings.

Downgrade incompatibilities with objc qualified types (e.g. id <P>) to warnings.
Note: One day, we should consider moving the actual diags to ObjCQualifiedIdTypesAreCompatible(), since it has more information on the actual problem. GCC currently emits slightly more instructive errors for some cases involving protocols. I added a FIXME to the code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57529 91177308-0d34-0410-b5e6-96231b3b80d8