Improve diagnostics with C++26 constexpr exceptions

[mingxwa]

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C++26 constexpr exceptions can also be considered here when the compiler implementations are widely available.

Relevant paper: https://wg21.link/p3557r0 (godbolt link: https://godbolt.org/z/Y1vPcn6Kr)

Adding constraints to the requires clause has the downside that when there are multiple overloads, the diagnostic messages are spammed with the "we tried this and that and none of them matched". constexpr exceptions don't have this issue.

Originally posted by @SidneyCogdill in #247