Inlining with nested classes

[odersky]

Inlining currently does not work very well if the inline method contains local classes. It works in simple cases but fails in many more complicated ones. Things to try: sealed class hierarchies, enums, generic classes.

It would be good to scan the issues for more concrete examples of what currently fails.

But in any case, what we would need is a systematic approach to copying classes and class hierarchies in inline methods. The tricky bit here is that we have to duplicate a bunch of symbols when inlining and we need to also update all possible references between such symbols.

[odersky]

Most problems should be addressed by #14537