[test] Split out Annotations from `TestingSupport`

The Annotations helper class does not have a gtest or gmock dependency, but because it's bundled with the rest of TestingSupport, it gets one. By splitting it out, a target can use it without being forced to use LLVM's copy of gtest.

Reviewed By: GMNGeoffrey, sammccall, gribozavr2

Differential Revision: https://reviews.llvm.org/D141175