[ASTImporter] Import member expr with explicit template args

Summary:
Member expressions with explicit template arguments were not imported
correctly: the DeclRefExpr was missing. This patch fixes.

Reviewers: a_sidorin, shafik, a.sidorin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58830

llvm-svn: 355596