Move unittest helpers to a shared location

Summary:
unittests/AST/Language.h defines some helpers that we would like to
reuse in other tests, for example, in tests for syntax trees.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: mgorny, martong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80792