Handle some more fallout with the conversion of using PointerType for

Objective-C pointers to using ObjCObjectPointerType.

Now the checking for 'attribute ((nonnull))' in Sema doesn't emit an error when
trying to apply that attribute to a parameter that is an Objective-C pointer
(this is a regression).

To prevent this regression from occuring in the future, the 'nonnull.c' test was
moved to test/SemaObjC and renamed 'nonnull.m'. I also enhanced the tests to
show that function calls involved a NULL Objective-C pointer constant does not
trigger a warning. This is consistent with GCC, but should likely be fixed.

llvm-svn: 75856