we still sometimes permit coercion into `!`

[nikomatsakis]

While looking into the precise behavior around ! for #47291, we realized that we still sometimes permit coercion into !, in the coerce_lub code:

rust/src/librustc_typeck/check/mod.rs

Lines 3541 to 3544 in 25749ad

	// Any expression that produces a value of type `!` must have diverged
	if ty.is_never() {
	self.diverges.set(self.diverges.get() | Diverges::Always);
	}

@arielb1 tried to remove that earlier #45880 but we missed this case.

[nikomatsakis]

cc #35121

[nikomatsakis]

It'd be nice to remove this paragraph from the comment on the diverges field, too:

    /// - To permit assignment into a local variable or other place
    ///   (including the "return slot") of type `!`.  This is allowed
    ///   if **either** the type of value being assigned is `!`, which
    ///   means the current code is dead, **or** the expression's
    ///   diverging flag is true, which means that a diverging value was
    ///   wrapped (e.g., `let x: ! = foo(return)`).

[nikomatsakis]

cc @varkor -- who may do this =)