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This upstreams a Google-internal change (141765019).

This upstreams a Google-internal change (146491438).

This merges a Google-internal change (117235625).

Original CL description:
This CL was created manually in about an hour with sed, a Python script
to find all the places unqualified 'string' was mentioned, and some help
from Emacs to add the "std::" qualifications, plus a few manual tweaks.

This guards use of <type_traits> and its features with
GTEST_HAS_STD_TYPE_TRAITS_, and std::function with
GTEST_HAS_STD_FUNCTION_.