[ASTImporter] Structural eq: handle DependentScopeDeclRefExpr

Summary:
Structural equivalence did not handle dependent template args properly
when the arg contained a DependentScopeDeclRefExpr.

Reviewers: a_sidorin, a.sidorin

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62329

llvm-svn: 364889