Fix equals method for value classes

Before this commit, the following class:
  class VC(val x: Int) extends AnyVal
resulted in the creation of the method:
  def equals(val x$0: Any): Boolean =
    x$0 match {
      case x$0 @ _: VC => true
      case _: Any => false
    }
After this commit, we get instead:
  def equals(val x$0: Any): Boolean =
    x$0 match {
      case x$0 @ _: VC => VC.this.a.==(x$0.a)
      case _: Any => false
    }

Author: smarter

src/dotty/tools/dotc/transform/SyntheticMethods.scala

@@ -49,7 +49,11 @@ class SyntheticMethods extends MiniPhaseTransform with IdentityDenotTransformer
    */
   def syntheticMethods(clazz: ClassSymbol)(implicit ctx: Context): List[Tree] = {
     val clazzType = clazz.typeRef
-    lazy val accessors = clazz.caseAccessors
+    lazy val accessors =
+      if (isDerivedValueClass(clazz))
+        clazz.paramAccessors
+      else
+        clazz.caseAccessors
 
     val symbolsToSynthesize: List[Symbol] =
       if (clazz.is(Case)) caseSymbols