Move _Char_traits_eq and _Char_traits_lt from <xstring> to <regex>

[frederick-vs-ja]

As these functors are only used in <regex>. Separated from #3623.

[StephanTLavavej]

Thanks, this is an excellent change!

• Pure code movement, no other changes.
• The partial specializations are unquestionably safe - while they aren't next to the primary templates, they appear immediately after the _Char_traits_meow types they're being specialized for, so there is zero chance that the primary templates could have been used before the partial specializations are seen.
• These are internal types, so there is no breaking change impact unless users were being very bad kitties indeed 😼

[barcharcraz]

Verified identical code (only movement). Looks good.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks again for this small but clean throughput improvement! 🐈 🚀 🎉