Checking in some code that is still under construction.

I need to (finally) change the way Class/id/SEL/IMP are built-in...the current approach of doing it in the preprocessor is "broken". The other problem is Sema::GetObjcIdType/GetObjcSelType/GetObjcClassType, the hooks that initialize ASTContext lazily. These built-in types need to be done up front...

llvm-svn: 43557