[Index] Prevent canonical decl becoming nullptr

Summary:
This patch prevents getCanonicalDecl returning nullptr in case it finds
a canonical TemplateDeclaration with no attached TemplatedDecl.
Found by running the indexer over a version of the standard library deep inside
a template metaprogramming mess.

Reviewers: klimek, vsk

Reviewed By: vsk

Subscribers: vsk, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D35212

llvm-svn: 308269