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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