Fix #8101: Change partially-augmented signalling

compiler/src/dotty/tools/dotc/transform/AugmentScala2Traits.scala

@@ -65,5 +65,6 @@ class AugmentScala2Traits extends MiniPhase with IdentityDenotTransformer { this
         sym.ensureNotPrivate.installAfter(thisPhase)
     }
     mixin.setFlag(Scala2xPartiallyAugmented)
+    mixin.transformAfter(thisPhase, d => { d.setFlag(Scala2xPartiallyAugmented); d })
   }
 }

tests/run/i8101/Foo.scala

@@ -0,0 +1,3 @@
+trait Foo {
+  def f: String = ???
+}

tests/run/i8101/JavaFoo.java

@@ -0,0 +1,3 @@
+public abstract class JavaFoo {
+    public abstract int read();
+}

tests/run/i8101/Test.scala

@@ -0,0 +1,6 @@
+class Bar extends JavaFoo with Foo {
+  def read(): Int = ???
+}
+
+@main def Test =
+  val stdout = new Bar