[c++20] consteval functions don't get vtable slots.

For the Itanium C++ ABI, this implements the rule added in
https://github.com/itanium-cxx-abi/cxx-abi/pull/83

For the MS C++ ABI, this implements the direction that seemed most
plausible based on personal correspondence with MSVC developers, but is
subject to change as they decide their ABI rule.