Implement P2652R2 Disallowing User Specialization Of allocator_traits

[frederick-vs-ja]

Fixes #3443. Also enables support and test coverage for EDG.

Upgrades allocator_traits::allocate_at_least to _NODISCARD_RAW_PTR_ALLOC.

I've filed DevCom-10301073 to request compiler support for this. Given compilers are already required to reject initializer_list specialization declarations, I think it's reasonable to do the same thing for allocator_traits.

[StephanTLavavej]

Thanks! 😻 I've pushed very minor changes, including a tiny throughput improvement. FYI @strega-nil-ms as you already approved.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks again for updating allocate_at_least()'s machinery, improving its nodiscard diagnostic, and requesting compiler support to keep bad kitties from jumping up on the table where they're not allowed! ⚙️ 🐈 😻