DebugInfo: Emit info for casted decls in template args

Summary:
Previously the backend wouldn't get to see the underlying GlobalValue
that corresponds to the template argument because it would be hidden by
a cast at the IR level.  Instead strip the pointer casts off of the
value until we see the underlying GlobalValue.

Reviewers: dblaikie, echristo, majnemer

Reviewed By: majnemer

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1508