Fix isInstantiated and isInTemplateInstantiation to not recreate the matchers on each call.

Summary:
Store the result matcher after the first call and reuse it later on.
Recreating the matchers just to use them once incurs in a lot of
unnecessary temporary memory allocations.
This change speeds up our clang-tidy benchmarks by ~2%.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6564

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223690 91177308-0d34-0410-b5e6-96231b3b80d8