[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@354262 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed