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

llvm-svn: 223690