[RFC] libstdc++: fix declaration and definition mismatch for _GLIBCXX20_CONSTEXPR

[Alexey Lapshin]

Thanks for the quick response,

I just want to save some space for embedded apps that are using c++
Related issue is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93008
So, I want to drop "constexpr" for templates.

I'm not familiar with the libstdc++ source code and coding style.
Is having mixed declarations and definitions intentional in libstdc++?

Regards,
Alexey