Generate static inline accessors in the outermost scope

[nicolasstucki]

If a class C needs inline accessors that would be added top-level or if the accessor is to a static member, we place it in a new invisible module C$inline$accessors.

If the accessor location in the new scheme is not the same as the previous location, we also generate the old accessor for backward binary compatibility but do not use it.

Fixes #13215
Fixes #15413

Example

package foo
class C:
  inline def baz: Unit = D.bazImpl
object D:
  private[foo] def bazImpl: Unit = ()

becomes

package foo
class C:
  inline def baz: Unit = foo.C$inline$accessors.inline$bazImpl:Unit
  // old accessor
  def inline$bazImpl$i1(x$0: foo.D): Unit = x$0.bazImpl
 
// new static accessors
object C$inline$accessors: 
  def inline$bazImpl: Unit = foo.D.bazImpl

object D:
  private[foo] def bazImpl: Unit = ()

[sjrd]

This ticket for this bug raised questions about binary compatibility. Is this fix actually compatible?

[nicolasstucki]

Yes, there are some issues with binary compatibility in this first implementation. I plan to generate the old (sometimes buggy) accessors for backward compatibility but not use them in the new code. I first need to create a set of representative cross-compilation tests.

This PR should not be merged if I do not manage to keep the binary compatibility.

[nicolasstucki]

Replaced with #16992