WIP [libc++][ranges] P3052R2: view_interface::at()

[H-G-Hristov]

Closes #204395

Implements https://wg21.link/P3052R2

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[Zingam]

@frederick-vs-ja IMO this one should be backported as DR but I don't see that this was proposed. WDYT?

Ref:
cplusplus/papers#1721
cplusplus/draft#9126

[frederick-vs-ja]

I haven't found any discussion about treating this as DR. Previously, we implemented span::at and mdspan::at as non-DR additions. If we want to DR-ify view_interface::at, IMO we should do the same for span and mdspan.

[Zingam]

I haven't found any discussion about treating this as DR. Previously, we implemented span::at and mdspan::at as non-DR additions. If we want to DR-ify view_interface::at, IMO we should do the same for span and mdspan.

If std::expected.has_error() is treated as DR, this one should be treated too. Anyway I'll park this until we get the C++29 support.

Thank you for clarifying.