[ASTImporter] Added visibility check for scoped enums.

Summary:
ASTImporter makes now difference between C++11 scoped enums with same
name in different translation units if these are not visible outside.
Enum declarations are linked into decl chain correctly.

Reviewers: martong, a.sidorin, shafik, a_sidorin, teemperor

Reviewed By: shafik, a_sidorin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74554