contrib/googletest: update from 1.14.0 to 1.15.2

The changes between the two versions can be found in this diff of the
two release tags:
https://github.com/google/googletest/compare/v1.14.0...v1.15.

contrib/googletest: update from 1.14.0 to 1.15.2

The changes between the two versions can be found in this diff of the
two release tags:
https://github.com/google/googletest/compare/v1.14.0...v1.15.2

One notable change is that GoogleTest 1.15.x now officially requires
C++-14 (1.14.x required C++-11).

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47197

Merge commit '14f7077fed7d82046bdcbe347004132f08aba886'

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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

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

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