Fix compilation of Google Test in C++20 mode

Without this patch when using CMAKE_CXX_STANDARD=20
and MSVC 19.30.30705.0 compilation of unit tests
fails with

llvm\utils\unittest\googlemock\include\gmock/gmock-actions.h(828): error C2039: 'result_of': is not a member of 'std'

Patch is taken from Google Test:
https://github.com/google/googletest/commit/61f010d703b32de9bfb20ab90ece38ab2f25977f

Do not use std::result_of as it was removed in C++20.

Differential Revision: https://reviews.llvm.org/D115163