move ObjCQualifiedIdTypesAreCompatible out of ASTContext into Sema.

While it is similar to the other compatibility predicates in ASTContext,
it is not used by them and is different.

In addition, greatly simplify ObjCQualifiedIdTypesAreCompatible and
fix some canonical type bugs.  Also, simplify my Type::getAsObjC* methods.

llvm-svn: 49313