Make prefix covariant in type maps.

According to the new subtyping rules,

    T <: U  ==>  T#X <: U#X and T#x <: U#x

Therefore type maps should treat the prefix of a named type
covariantly.

Author: odersky

src/dotty/tools/dotc/core/Types.scala

@@ -2884,13 +2884,7 @@ object Types {
       tp match {
         case tp: NamedType =>
           if (stopAtStatic && tp.symbol.isStatic) tp
-          else {
-            val saved = variance
-            variance = 0
-            val result = tp.derivedSelect(this(tp.prefix))
-            variance = saved
-            result
-          }
+          else tp.derivedSelect(this(tp.prefix))
 
         case _: ThisType
           | _: BoundType