Allow pattern matching to constrain type members of function arguments

[Blaisorblade]

Simplifying from @b-studios #4376 (comment):

scala> object existentials {
     |
     |   sealed trait Exists { type R }
     |   type Rank2[F[_]] = (e: Exists) |=> F[e.R]
     |
     |   type ~>[F[_], G[_]] = Rank2[[X] => F[X] => G[X]]
     |
     |   val nat: List ~> Option = {
     |     case Nil => None
     |     // also: x isn't refined to e.R
     |     case x :: _ => Some(x)
     |   }
     | }
11 |    case x :: _ => Some(x)
   |                        ^
   |                        Found:    Any(x)
   |                        Required: evidence$1.R

Since @abgruszecki is working on GADT type refinement and using singletons, we might make progress. Since @milessabin recently said he's fixing bugs with higher-kinded types, tagging him too.

[abgruszecki]

Tentatively classifying under "enhancement". Might be doable if we make term symbols valid arguments for GADTMap. #5747 is quite likely relevant, since it allows constraining singleton types with null-checks.