Bugfix: Relax experimental inheritance criterion

We previously required that a class inheriting an @experimental class is itself
@experimental. This is too strict. It should be OK to have the inheriting class
be contained in an experimental scope. I.e.
```
@experimental class A
@experimental object o:
  class B extends A
```
is now OK.

Author: odersky

compiler/src/dotty/tools/dotc/typer/CrossVersionChecks.scala

@@ -117,7 +117,7 @@ class CrossVersionChecks extends MiniPhase:
 
   /** Check that classes extending experimental classes or nested in experimental classes have the @experimental annotation. */
   private def checkExperimentalInheritance(cls: ClassSymbol)(using Context): Unit =
-    if !cls.isAnonymousClass && !cls.hasAnnotation(defn.ExperimentalAnnot) then
+    if !cls.isAnonymousClass && !cls.isInExperimentalScope then
       cls.info.parents.find(_.typeSymbol.isExperimental) match
         case Some(parent) =>
           report.error(em"extension of experimental ${parent.typeSymbol} must have @experimental annotation", cls.srcPos)

tests/pos/experimental-inheritance.scala

@@ -0,0 +1,4 @@
+import annotation.experimental
+@experimental class A
+@experimental object o:
+  class B extends A