Initial patch to support definitions of id and Class from headers in Objective-C code.

This currently breaks test/SemaObjC/id-isa-ref.m and issues some spurious warnings when you attempt to assign a struct objc_class* value to a Class variable.  The test case probably should fail as it's written, because without the definition of Class the compiler should not assume struct objc_class* is a valid receiver type, but it's left broken because it would be nice if we could get that passing too for the special case of isa.

Approved by snaroff.

llvm-svn: 79248