Add missing dereferenceable_or_null getters

Summary: Add missing dereferenceable_or_null getters required for
http://reviews.llvm.org/D9253 change. Separated from the D9253 review.

Patch by Artur Pilipenko!

Reviewers: sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9499

llvm-svn: 236615