Generate static forwarders for object members in companion interface

Same change implemented five years ago in Scala 2:
scala/scala#5131

Fixes scala#13572.

Author: smarter

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -292,7 +292,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
       emitAnnotations(cnode, claszSymbol.annotations ++ ssa)
 
       if (!isCZStaticModule && !isCZParcelable) {
-        val skipStaticForwarders = (claszSymbol.isInterface || claszSymbol.is(Module) || ctx.settings.XnoForwarders.value)
+        val skipStaticForwarders = (claszSymbol.is(Module) || ctx.settings.XnoForwarders.value)
         if (!skipStaticForwarders) {
           val lmoc = claszSymbol.companionModule
           // add static forwarders if there are no name conflicts; see bugs #363 and #1735

tests/run/trait-static-forwarder.check

@@ -0,0 +1 @@
+42

tests/run/trait-static-forwarder/Test.java

@@ -0,0 +1,5 @@
+public final class Test {
+    public static void main(String... args) {
+        System.out.println(T.foo());
+    }
+}

tests/run/trait-static-forwarder/forwarders.scala

@@ -0,0 +1,5 @@
+trait T
+
+object T {
+  def foo = 42
+}