Generate static forwarders for object members in companion interface

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

Fixes #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

compiler/test/dotc/run-test-pickling.blacklist

@@ -1,3 +1,6 @@
+## Many of these tests fail because CompilationTests.pickling does not handle
+## tests containing java files correctly
+
 derive-generic.scala
 eff-dependent.scala
 enum-java
@@ -36,4 +39,5 @@ zero-arity-case-class.scala
 i12194.scala
 i12753
 t6138
-t6138-2
+t6138-2
+trait-static-forwarder

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
+}