[ASTImporter] Added visibility check for variable templates.

Summary:
ASTImporter makes now difference between variable templates
with same name in different translation units if not visible
outside.

Reviewers: a.sidorin, shafik, a_sidorin

Reviewed By: a_sidorin

Subscribers: dkrupp, Szelethus, gamesh411, teemperor, martong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75732