Maintain Extension flags for forwarders when creating an anonymous class.
Maintain Extension flags for forwarders when creating an anonymous class.

This causes an error in RefChecks for run/fragables-extension.scala otherwise. Previously
the error was not detected since methods created in the same group as RefChecks are not checked.

dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty