C++: Actually check function names in MaD

[MathiasVP]

This PR fixes quite an embarrassing bug: We forgot to check the function name when matching MaD models with Functions when dealing with non-member functions 😱

That's ... quite bad. Luckily, there are several reasons for why this went unnoticed for 4 months:

• The only non-member models we have is boost::asio::buffer and std::format
• We were still correctly validating the signature, which meant that very few spurious functions were being matched.

Commit-by-commit review recommended. The last commit is just a cleanup of the conjuncts.

[geoffw0]

We were still correctly validating the signature, which meant that very few spurious functions were being matched.

It looks like we were also validating the name for non-member functions where we didn't provide an [optional] signature as well, but there was indeed a rather large issue with non-member functions where a signature was provided.

[geoffw0]

LGTM.