Fix IncompatibleClassChangeError under Java 9

[smarter]

The itf flag of visitMethodInsn and Handle in ASM needs to be true
when the method is defined in an interface. Java 8 ignores this but Java
9 fails at runtime with:
java.lang.IncompatibleClassChangeError: Method ... must be InterfaceMethodref constant

This commit is inspired by similar changes in scalac, in particular see
7d51b3f by Jason Zaugg
(from scala#5251) and
e619b03 by Lukas Rytz
(from scala#5293). This issue was also
discussed in scala#5452 (which does not
matter for Dotty since we do not run the backend optimizer).

[smarter]

Corresponding Dotty PR: scala/scala3#3138

[smarter]

@retronym @lrytz Maybe you have some idea on the root cause requiring 48e0589. How does Scalac handle static methods on interfaces? Is the receiver the companion object created by ClassfileParser too?

[retronym]

Scalac 2.12.3 emits an InterfaceMethodRef

⚡ (java_use 9; cat sandbox/test.scala; scalac sandbox/test.scala && javap -c C.class; java Test )
object Test { def main(args: Array[String]): Unit = java.util.List.of("", "") }

Compiled from "test.scala"
public class C {
  public void main(java.lang.String[]);
    Code:
       0: ldc           #13                 // String
       2: ldc           #13                 // String
       4: invokestatic  #19                 // InterfaceMethod java/util/List.of:(Ljava/lang/Object;Ljava/lang/Object;)Ljava/util/List;
       7: pop
       8: return

  public C();
    Code:
       0: aload_0
       1: invokespecial #26                 // Method java/lang/Object."<init>":()V
       4: return
}

See:

https://github.com/scala/scala/blob/32f597704698291b7162335844b366a20873c67b/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala#L332-L335

https://github.com/scala/scala/blob/3347b806adecfbf663e7db4e1962504d9b106b83/src/compiler/scala/tools/nsc/backend/jvm/BTypes.scala#L677

https://github.com/scala/scala/blob/3347b806adecfbf663e7db4e1962504d9b106b83/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala#L1077-L1098

[smarter]

Thanks! I guess this answers my question: https://github.com/scala/scala/blob/32f597704698291b7162335844b366a20873c67b/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala#L76-L79, you're calling isInterface on a representation which already takes care of getting rid of companion objects.