Merge topic 'clangcl-fixes'

091afa7342 Tests: Teach tests when to treat clang-cl as MSVC
006768903c Work around clang-cl breakage on make_unique/std::forward
d625dfcdf9 Avoid compiling CMake itself as C++17 with Clang's MSVC ABI

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !2573