DebugInfo: Emit info for constant expressions in template arguments

Summary:
This allows us to handle the general case where a non-type template
argument evaluates to a constant expression which isn't integral or a
declaration.

This fixes PR16939.

Reviewers: dblaikie, rsmith

Reviewed By: dblaikie

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1453