[Bug libstdc++/124444] [16 Regression] debug _M_invalidate_if incompatible with C++20 since r16-5845

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124444

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #0)
> Wonder if for C++20 the body doesn't have to be moved to a non-constexpr
> function template.

I think we could just put the `if (std::__is_constant_evaluated())` checks back
on the caller side, instead of inside the _M_invalidate_if() function.