[AST matchers] Add basic matchers for googletest EXPECT/ASSERT calls.

Summary:
This revision adds matchers that match calls to the gtest EXPECT and ASSERT
macros almost like function calls. The matchers are placed in separate files
(GtestMatchers...), because they are specific to the gtest library.

Reviewers: gribozavr2

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74840