stabilize const_ptr_is_null

[RalfJung]

FCP passed in #74939.

The second commit cleans up const stability around UB checks a bit, now that everything they need (except for const_eval_select) is stable.

Fixes #74939

[rustbot]

r? @tgross35

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[rustbot]

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

cc @rust-lang/miri, @rust-lang/wg-const-eval

[rustbot]

Some changes occurred to the CTFE machinery

cc @rust-lang/wg-const-eval

[dtolnay]

@bors r+

[bors]

📌 Commit 543627d has been approved by dtolnay

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.