Generate opaque proxies also for base classes

[odersky]

Fixes #14656

[prolativ]

Just to make this clear (because the name of the PR made me wonder):
This PR does NOT intend to make an opaque type transparent in the scopes of subtypes of a class/trait defining the opaque type, right? Because that would change the current language specification if I got this right.
https://docs.scala-lang.org/scala3/reference/other-new-features/opaques-details.html

Inside the scope of the alias definition, the alias is transparent: T is treated as a normal alias of R. Outside its scope, the alias is treated as the abstract type

[odersky]

Good point! Yes, we have to modify this logic. We have to only create proxies for the base class where the inline method is located, not for all of them.

[odersky]

Correction: We already only take opaque types into account that come from a class enclosing the inlined method. So that looks correct to me.