[PATCH] libstdc++: add range support to std::optional (P3168)

[Giuseppe D'Angelo]

Hello,

The attached patch implements C++26's range support for std::optional.

It's also available on Forge here:

https://forge.sourceware.org/gcc/gcc-TEST/pulls/54

It's mostly straightforward, but I had to lift format_kind out of 
<format> (together with a few other helpers) to avoid <optional> to 
depend on that. I put it in formatfwd.h, but maybe it ought to go in its 
own header?


Thank you,

-- 
Giuseppe D'Angelo
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