Don't force denotations in asMemberOf

This was introduced in the previous commit and caused unpickling
failures, we are now more conservative and only check the Private flag
on SymDenotations so we don't have to load any other denotation.

Author: smarter

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

@@ -1585,11 +1585,12 @@ object Types {
 
     protected def asMemberOf(prefix: Type)(implicit ctx: Context): Denotation =
       if (name.isShadowedName) prefix.nonPrivateMember(name.revertShadowed)
-      else {
-        val d = lastDenotation
-        // Never go from a non-private denotation to a private one
-        if (d == null || d.symbol.is(Private)) prefix.member(name)
-        else prefix.nonPrivateMember(name)
+      else lastDenotation match {
+        case d: SymDenotation if !d.is(Private) =>
+          // We shouldn't go from a non-private denotation to a private one
+          prefix.nonPrivateMember(name)
+        case _ =>
+          prefix.member(name)
       }
 
     /** (1) Reduce a type-ref `W # X` or `W { ... } # U`, where `W` is a wildcard type