Follow-up to D6217

Summary:
Ok, here is somewhat addition to D6217 aiming to preserve old darwin behavior wrt the typedefed types. The actual change to SemaChecking turned out to be pretty gross, in particular:
  1. We need to extract the typedef'ed type for proper diagnostics
  2. We need to walk over paren expressions as well

Reviewers: chandlerc, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6256