Allow concrete private interface methods in Java sources

[dengziming]

Java 9+ allows concrete private interface methods. This PR makes scalac's Java parser accept them.

fixes scala/bug#12393

[SethTisue]

changing isConcreteInterfaceMethod affects isBodyOk, which is the desired effect

but then isConcreteInterfaceMethod is used again later, as follows:

          // for abstract methods (of classes), the `DEFERRED` flag is already set.
          // here we also set it for interface methods that are not static and not default.
          if (!isConcreteInterfaceMethod) mods1 |= Flags.DEFERRED

is the change you have made correct for this use of isConcreteInterfaceMethod as well?

[lrytz]

is the change you have made correct for this use of isConcreteInterfaceMethod as well?

Seems correct