Untag dispatch <xstring>

[AlexGuteniev]

Towards #189

Same as #2479, nothing to see here

[StephanTLavavej]

Looks great, thanks for the continued if constexprization!

I pushed a trivial change to remove // general algorithm comments (matching your previous _Traits_find_first_of() change) - these were the counterparts of the // special case for std::char_traits comments, and both are no longer necessary now that there's a clear if constexpr (_Special) { ... } else { ... }.

Note to reviewers: as before, GitHub makes this diff look way worse than it really is. I copied each pair of old and new implementations into VS Code editors, then reordered the old overloads so that the "special" case occurs first. Along with a bit of indentation, this made the diff easy to read.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for completing the untag dispatching of <xstring>'s find functions and improving throughput! 🚀 🐇 ⏱️