Add template type and value parameter metadata nodes to template variable specializations

Summary: Add an optional attribute referring to a tuple of type and value template parameter nodes to the DIGlobalVariable node. This allows us to record the parameters of template variable specializations.

Reviewers: dblaikie, aprantl, probinson, JDevlieghere, clayborg, jingham

Reviewed By: JDevlieghere

Subscribers: cfe-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D52058

llvm-svn: 343707