_Fake_copy_init unification

[StephanTLavavej]

• Replace _Implicitly_convert_to and _Fake_decay_copy with _Fake_copy_init.
  • Followup to Mark several functions as noexcept. #1937.
• Adjust warning suppressions.
  • In <type_traits>, _Is_nothrow_convertible_v isn't declaring any functions, so we don't need to suppress the volatile deprecation warnings about WG21-N4910 [depr.volatile.type]/3.
  • In <xstddef>, we're just declaring _Fake_copy_init() but not calling it, so we don't need to suppress truncation/sign-conversion warnings.
  • However, _Fake_copy_init() returns _Ty, unlike _Implicitly_convert_to which returned void. There's a different warning number about volatile return types that we need to suppress.

As mentioned in #1937's comments, this return type change from void to _Ty has no other effects - the only usage outside of noexcept is already using void_t:

STL/stl/inc/type_traits

Line 1637 in 20db59e

struct _Invoke_convertible<_From, _To, void_t<decltype(_Implicitly_convert_to<_To>(_Returns_exactly<_From>()))>>

[strega-nil-ms]

Thanks Stephan!

[StephanTLavavej]

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

[CaseyCarter]

Thank you for clarifying what's a real copy and a fake copy. No more will we be forced to say things like "It's a fake copy, innit?"