[CXX] Templates specialization visibility can be wrong

Summary:
Under some conditions, LinkageComputer can get the visibility for
ClassTemplateSpecializationDecl wrong because it failed to find the Decl
that has the explicit visibility.

This fixes:
llvm.org/bugs/pr36810
rdar://problem/38080953

Reviewers: rsmith, arphaman, doug.gregor

Reviewed By: doug.gregor

Subscribers: doug.gregor, cfe-commits

Differential Revision: https://reviews.llvm.org/D44670

llvm-svn: 330338