[ObjC] Messages to 'self' in class methods that return 'instancetype' should

use the pointer to the class as the result type of the message

Prior to this commit, messages to self in class methods were treated as instance
methods to a Class value. When these methods returned instancetype the compiler
only saw id through the instancetype, and not the Interface *. This caused
problems when that return value was a receiver in a message send, as the
compiler couldn't select the right method declaration and had to rely on a
selection from the global method pool.

This commit modifies the semantics of such message sends and uses class messages
that are dispatched to the interface that corresponds to the class that contains
the class method. This ensures that instancetypes are correctly interpreted by
the compiler. This change is safe under ARC (as self can't be reassigned),
however, it also applies to MRR code as we are assuming that the user isn't
doing anything unreasonable.

rdar://20940997

Differential Revision: https://reviews.llvm.org/D36790

llvm-svn: 349841