Support for constexpr-ness and noexcept-ness of underlying interface

[Twon]

Right now, the generated protocol type does not account for the constexpr (including consteval where applicable) and noexception status of the underlying interface. Support in the generator should be extended to propagate this information.

[jbcoe]

This might be worth doing in 2 PRs

[jbcoe]

https://github.com/jbcoe/py_cppmodel.git needs updating to support constexpr.

Update: addressed in jbcoe/py_cppmodel#24
Update (2): libclang patch proposed llvm/llvm-project#188965 supersetted by llvm/llvm-project#183305

[jbcoe]

constexpr support needs a later C++ standard than C++20.

noexcept is supported by #37