[libc] [UnitTest] Add Matchers

Summary: This patch adds gtest-like matchers and `EXPECT|ASSERT_THAT` macros. It also adds matchers `Succeeds` and `Fails` and has examples using these in test/src/signal/sigaddset_test.cpp.

Reviewers: sivachandra, gchatelet, PaulkaToast

Reviewed By: sivachandra, PaulkaToast

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D75487