Refactor FileCheck to make it usable as an API

https://reviews.llvm.org/D50283
reviewed by bogner

This patch refactors FileCheck's implementation into support so it can
be used from C++ in other places (Unit tests).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339192 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed