implement LWG-3471: polymorphic_allocator::allocate does not satisfy Cpp17Allocator requirements

[fsb4000]

Fixes #2550

I saw that the changes were in "Previous resolution [SUPERSEDED]" but as far as I understand the new resolution is needed because of changing the wording and the proposed implementation stays the same.

std::launder is in <new>

<xmemory> includes <new>

<xpolymorphic_allocator.h> includes <xmemory>

[timsong-cpp]

The original wording is incorrect (and the launder has no effect - laundering a pointer to byte that is already pointing to a live std::byte accomplishes precisely nothing).

The intended implementation (taking advantage of [intro.object]/13) is

    return ::operator new(_Bytes, _Ptr);

[CaseyCarter]

I can't imagine how we would test for this, so I won't ask for test coverage.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. (I'll handle verifying that this works with the recent LLVM update.)

[StephanTLavavej]

Thanks for implementing another LWG issue resolution! ✅ 💯 🎉