First step toward fixing <rdar://problem/6613046> refactor clang objc type representation.

Add a type (ObjCObjectPointerType) and remove a type (ObjCQualifiedIdType).

This large/tedious patch is just a first step. Next step is to remove ObjCQualifiedInterfaceType. After that, I will remove the magic TypedefType for 'id' (installed by Sema). This work will enable various simplifications throughout clang (when dealing with ObjC types). 

No functionality change.

llvm-svn: 73649