Also allow structural dispatch on Dynamic

Given
```scala
trait Sel extends Dynamic

extension (s: Sel)
  def selectDynamic(name: String) = ???
```
the following worked:
```scala
val sel = new Sel {}
val foo = sel.foo
```
but the following didn't:
```scala
val sel2 = (new Sel {}).asInstanceOf[Sel{ def foo: String }]
val foo2 = sel2.foo
```
The problem was that we recognized a structural dispatch and then required
the qualifier to be an instance of `Selectable`. But in fact, `Dynamic`
works just as well, and the mechanism is the same. It's just that
`Dynamic` is less type safe then `Selectable`.

compiler/src/dotty/tools/dotc/typer/Dynamic.scala

@@ -181,7 +181,7 @@ trait Dynamic {
     val vargss = termArgss(tree)
 
     def structuralCall(selectorName: TermName, classOfs: => List[Tree]) = {
-      val selectable = adapt(qual, defn.SelectableClass.typeRef)
+      val selectable = adapt(qual, defn.SelectableClass.typeRef | defn.DynamicClass.typeRef)
 
       // ($qual: Selectable).$selectorName("$name")
       val base =

tests/pos/i17100a.scala

@@ -0,0 +1,12 @@
+
+import scala.language.dynamics
+trait Sel extends Dynamic
+
+extension (s: Sel)
+  def selectDynamic(name: String) = ???
+
+val sel = new Sel {}
+val foo = sel.foo
+val sel2 = (new Sel {}).asInstanceOf[Sel{ def foo: String }]
+val foo2 = sel2.foo
+