Use libClangTesting in the unittest for AST matchers

Summary:
The unittest for AST matchers has its own way to specify language
standards. I unified it with the shared infrastructure from
libClangTesting.

Reviewers: jdoerfert, hlopko

Reviewed By: hlopko

Subscribers: mgorny, sstefan1, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81150