Disallow use of PolyFunction in user code

[nicolasstucki]

The PolyFunction trait should only be used for compiler generated
encoded lambdas.

Any other use case that was allowed before was accidental. In the future,
we might consider supporting these if there is a good use case. This would
probably require a SIP.

Fixes #10075

[smarter]

I think this might be too drastic, PolyFunction could come up in type inference for example (val x = if cond then polyFun1 else polyFun2), and if a type can be inferred it should be expressible whenever possible. An alternative I would favor would be to mark PolyFunction as @deprecatedInheritance.

[smarter]

(I'm assuming we can tweak @deprecatedInheritance to not warn for poly function syntax, just explicit subclasses)

[nicolasstucki]

We do not seem to be checking for deprecatedInheritance (see #19002).

[nicolasstucki]

We could use the first commit (1b4b390) instead. This one only checks the parents of classes and does not restrict anything on types.

[smarter]

Using just the first commit sounds good to me.

[nicolasstucki]

Using just the first commit sounds good to me.

Removed the second commit and rebased