AST: Fix visibility calculation for VarTemplateSpecializationDecl

It is possible that a variable template specialization might not have a
VisibilityAttr attached to it while the template that it specializes
does, in fact, have one.

We should consider the template in such cases.

This fixes PR19597.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207498 91177308-0d34-0410-b5e6-96231b3b80d8