ASTMatchers: Add a matcher to detect whether a decl or stmt is inside a template instantiation.

This is hoisted from clang-tidy where it's used everywhere. The implementation
is not particularly efficient right now, but there is no easy fix for that.

Differential Revision: http://reviews.llvm.org/D5085

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217029 91177308-0d34-0410-b5e6-96231b3b80d8