Report illegal capture reference only once

If we do not strip the capture set at this point,
then the type will be transformed twice and
we'll get the same error message twice.

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -328,7 +328,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
                 CapturingType(parent2, ann.tree.toCaptureSet)
               catch case ex: IllegalCaptureRef =>
                 report.error(em"Illegal capture reference: ${ex.getMessage.nn}", tptToCheck.fold(NoSourcePosition)(_.srcPos))
-                t
+                parent2
             else
               t.derivedAnnotatedType(parent1, ann)
           case throwsAlias(res, exc) =>