[Clang][ASTImporter] Added visibility check for ClassTemplateDecl.

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

Reviewers: martong, a.sidorin, shafik

Reviewed By: martong

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67543