When declaring an ObjC interface decl with a @compatibility_alias alias name, change the class name to the "real" one.

If we have something like

  @class NewImage;
  @compatibility_alias OldImage NewImage;
  @class OldImage;

the lookup for 'OldImage' will return the 'NewImage' decl ("@class NewImage").
In such a case, when creating the decl for "@class OldImage" use the real declaration name ("NewImage"),
instead of the alias one ("OldImage"), otherwise we will break IdentifierResolver and redecls-chain invariants.

Fixes crash of rdar://14112291.
We want a string representation of the attribute, not the kind as a string.