fix: update type_to_name for Layout builder cxx_14 target

[ianna]

• temporary replacement helper variable templates for these that are part of C++17
• char specialisation of type_to_name needed for Layout builder to generate a correct form

[codecov]

Codecov Report

Merging #2165 (d5076b3) into main (37547b0) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

[jpivarski]

I'm in favor of this. Our current standard is C++14, so if std::is_xyz_v<T> is a C++17 thing, then we should define our own.

(Our C++ standard is constrained by the Docker images that pip uses to build wheels for many-Linux targets: they're tied to particular versions of gcc. I don't want to get into adding "download a new gcc" as a step in the wheel-building procedure; dependence on external downloads makes a build procedure more brittle.)