Make skolems uncached

There was no reason in the first case to cache them, as their
equality is reference identity. Maybe this fixes the OOM errors
we encounter.

Author: odersky

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

@@ -2506,11 +2506,11 @@ object Types {
   // ----- Skolem types -----------------------------------------------
 
   /** A skolem type reference with underlying type `binder`. */
-  abstract case class SkolemType(info: Type) extends CachedProxyType with ValueType with SingletonType {
+  abstract case class SkolemType(info: Type) extends UncachedProxyType with ValueType with SingletonType {
     override def underlying(implicit ctx: Context) = info
     def derivedSkolemType(info: Type)(implicit ctx: Context) =
       if (info eq this.info) this else SkolemType(info)
-    override def computeHash: Int = identityHash
+    override def hashCode: Int = identityHash
     override def equals(that: Any) = this eq that.asInstanceOf[AnyRef]
     override def toString = s"Skolem($info)"
   }