Disallow inline methods and opaque types in same scope

[odersky]

It looks extraordinarily hard to me to mix opaque types and inline methods. Combining inline methods with private members was already hard, see PrepareInlineable.scala which deals with this. Combining
inline methods with opaque types will be much harder. Quoting from #6846:

We cannot simply move
code compiled inside an opaque alias scope to outside this scope. The compiler can try to insert
some casts to make it work, but I see no general proven way to do so, because opaque types are
explained through type abstraction instead of through implicit conversions.

My current guess is that I'd need 2-4 weeks of full time work to arrive at a conclusion whether it's possible or not (I already spent 3 full days to arrive at this conclusion). But I don't have the time to do this. So it's safer we forbid this combination in an implementation restriction.

This PR also integrates the bits from #6846 that were worth salvaging.