[libclang] Visit attributes for function and class templates

Summary: Previously, `VisitAttributes` was not called for function and class templates and thus their attributes were not accessible using libclang.

Reviewers: bkramer, arphaman, rsmith, jbcoe

Reviewed By: jbcoe

Subscribers: cfe-commits

Tags: #clang

Patch by jklaehn (Johann Klähn)

Differential Revision: https://reviews.llvm.org/D36955

llvm-svn: 315958