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

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

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/D67490

llvm-svn: 371820