[clang][Index] Fix usage of IndexImplicitInstantiation

Summary:
Indexing context was skipping explicit template instantiations as well.
This patch makes sure it only skips implicit ones.

Subscribers: arphaman, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58189

llvm-svn: 354262