Import GoogleTest 1.14.0

GoogleTest 1.14.0 now requires C++14 to build. Change
`googletest.test.inc.mk` to reflect this requirement.

Adjust the build integration logic to handle the new version of
GoogleTest (add/remove headers/sources as needed).

Tighten down warnings via `CXXFLAGS.clang` instead of ignoring all
warnings. Some new warnings snuck in after I did my last round of fix
submissions upstream.

Also address some overlinking added in the previous version import by
removing superfluous libraries.

===============================

Expect WhenDynamicCastToTest.AmbiguousCast to fail

This change reapplies the expected failure from 1.10.0.

Ref: https://github.com/google/googletest/issues/2172

MFC after: 2 weeks
Reviewed by: asomers, emaste
Differential Revision: https://reviews.freebsd.org/D41399

Merge commit '8ef491440fcaec96f899d73e08873426c78583a4' into googletest-v1.14.0-import

googletest: pick from upstream: Don't allow signed/unsigned wchar_t in gcc 9 and later

Pick 711fccf8317b4fb7adc21c00fc1e20823c5d875f from upstream googletest:

Don't allow signed/unsigned wchar_t in gcc 9 and later

Upstream pull request: https://github.com/google/googletest/pull/2270

Sponsored by: Dell EMC Isilon