Add @implicitAmbiguous annotation

[puffnfresh]

https://issues.scala-lang.org/browse/SI-6806

Should be pretty straightforward.

Motivation:

trait =!=[A, B]

implicit def neq[A, B] : A =!= B = null

@implicitAmbiguous("Type is required to not equal ${A}")
implicit def neqAmbig1[A] : A =!= A = null
implicit def neqAmbig2[A] : A =!= A = null

[propensive]

We should consider how this might interact with #25.

As I see it, it ought to be fine -- it's just the "other reason" why implicit search may fail, and the @implicitAmbiguous error messages should be shown in the list too.

Tell me if I'm oversimplifying this.

[milessabin]

👍

[puffnfresh]

We should be able to give a nice error for any2stringadd but currently the annotation is not used in the right spot. That should be an easy fix.

[SethTisue]

merged in main repo: scala#4673