[ASTImporter] Added visibility context check for EnumDecl.

Summary:
ASTImporter makes now difference between enums with same name in different translation
units if these are not visible outside.
("Scoped enums" are not handled yet.)

Reviewers: martong, a.sidorin, shafik, a_sidorin

Reviewed By: a_sidorin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62484