Fix gap in const stability checks around intrinsics

[RalfJung]

@N1ark managed to find a gap in our const stability checks around intrinsics. This PR fixes that gap.

Sadly the standard library already relies on the gap, so for now I added a rustc_allow_const_fn_unstable but that attribute usually needs some process to be approved.

[rustbot]

⚠️ #[rustc_allow_const_fn_unstable] needs careful audit to avoid accidentally exposing unstable
implementation details on stable.

cc @rust-lang/wg-const-eval

Some changes occurred to constck

cc @fee1-dead

Some changes occurred to the CTFE machinery

cc @oli-obk, @lcnr

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @oli-obk, @lcnr

[rustbot]

r? @mati865

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[N1ark]

sorry about that 😓

[RalfJung]

sorry about that 😓

It was me who introduced this bug, you just found it. ;)

[mati865]

@rustbot reroll