[ASTImporter] Using Lang_CXX14 in ASTImporterVisibilityTest.

Summary:
These tests may work with C++14 language constructs in the future
(variable templates and others).
To avoid warnings about language version C++ version constants in the tests
are updated.

Reviewers: martong, a.sidorin

Reviewed By: martong

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64477

llvm-svn: 366061