Backport "Fix #20897: Make Nothing ⋔ Nothing, as per spec." to 3.5.2
#21458
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`derivesFrom`, used in `provablyDisjointClasses`, normally returns `false` when the receiver is `Nothing`. However, it returns `true` if the right-hand-side happens to be exactly `Nothing` as well. For the purpose of computing `provablyDisjoint`, that is not what we want. The root issue was that we let the previous algorithm handle `Nothing` like a class type, which it *is* in dotc but not in the spec. That led to this mistake. `AnyKind` suffers a similar issue, but already had special-cases in various places to mitigate it. Instead of adding a new special-case for `Nothing` inside `provablyDisjointClasses`, we address the root issue. Now we deal with `Nothing` and `AnyKind` early, before trying any of the code paths that handle (real) class types. [Cherry-picked b7846c4]
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Backports #21241 to the 3.5.2 branch.
PR submitted by the release tooling.
[skip ci]