Implement LWG-3436 std::construct_at should support arrays

[frederick-vs-ja]

Fixes #5110.

Currently, all tested compilers (MSVC, Clang, and EDG) need some workaround for constant evaluation.

• MSVC is blocked by lack of [[msvc::constexpr]] of the involved operator new[] in VCRuntime.
  • I'm attempting to introduce a new operator new[] overload, which is sufficiently constrained to avoid affecting overload resolution, as a workaround. However, it doesn't work for non-trivially destructible element types, because MSVC will require additional space to record the number of class objects to destroy.
• Clang crashes when directly using the final resolution of LWG-3436.
  • The old proposed resolution is used as a workaround.
• EDG simply can't accept the final resolution in constant evaluation.
  • The old proposed resolution is used as a workaround.
• Also, EDG doesn't correctly recognize the allocated element type of std::allocator<T[N]>::allocate in constant evaluation.
  • Workaround is unknown, so such case isn't made supported for EDG yet.
• Also, EDG crashes in some cases in constant evaluation.
  • Workaround is unknown, so such case isn't made supported for EDG yet.

Reported issues:

• DevCom-10798069 for VCRuntime. Not sure whether there are other bugs in the C1XX frontend.
• LLVM-117294 for Clang. Not sure whether there are other bugs.
• DevCom-10798145, DevCom-11008487, and DevCom-11012299 for EDG.

[StephanTLavavej]

Thanks for reporting all of these compiler bugs, I really appreciate it! 😻

[StephanTLavavej]

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

[StephanTLavavej]

⚙️ 😻 🎁