[ASTImporter] Added visibility context check for CXXRecordDecl.

Summary:
ASTImporter makes now difference between classes with same name in different
translation units if these are not visible outside. These classes are not linked
into one decl chain.

Reviewers: martong, a.sidorin, shafik

Reviewed By: shafik

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62312