Allow capture conversion after typer

It seems this is necessary for code generation. We got abstract method errors in
typer/TermRefSet otherwise:

```
 java.lang.AbstractMethodError: Method dotty/tools/dotc/typer/TermRefSet$$anon$1.accept(Ljava/lang/Object;Ljava/lang/Object;)V is abstract, took 2.601 sec
[error]     at dotty.tools.dotc.typer.TermRefSet$$anon$1.accept(Implicits.scala)
[error]     at java.util.LinkedHashMap.forEach(LinkedHashMap.java:684)
[error]     at dotty.tools.dotc.typer.TermRefSet.foreach(Implicits.scala:1638)
[error]     at dotty.tools.dotc.typer.TermRefSet.$plus$plus$eq(Implicits.scala:1634)
```

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1041,12 +1041,12 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
             arg1 match {
               case arg1: TypeBounds =>
 	            tparam match {
-	              case tparam: Symbol if leftRoot.isStable =>
+	              case tparam: Symbol if leftRoot.isStable || ctx.isAfterTyper =>
 	                val captured = TypeRef(leftRoot, tparam)
 	                isSubArg(captured, arg2)
 	              case _ =>
 	                false
-	            }        
+	            }
               case _ =>
                 (v > 0 || isSubType(arg2, arg1)) &&
                 (v < 0 || isSubType(arg1, arg2))