std: Ensure AssertRecoverSafe indeed is more often

Types like `&AssertRecoverSafe<T>` and `Rc<AssertRecoverSafe<T>>` were
mistakenly not considered recover safe, but the point of the assertion wrapper
is that it indeed is! This was caused by an interaction between the
`RecoverSafe` and `NoUnsafeCell` marker traits, and this is updated by adding an
impl of the `NoUnsafeCell` marker trait for `AssertRecoverSafe` to ensure that
it never interacts with the other negative impls of `RecoverSafe`.

cc #30510

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