Generate static forwarders for object members in companion interface

We used to disable generation of static forwarders when a object had a
trait as a companion, as one could not add methods with bodies to an
interface in JVM 6.

The JVM lifted this restriction to support default methods in interfaces,
so we can lift the restriction on static forwarders, too.

I've only commited the test using the default backend (GenBCode), but
I've also made the change and manaully verified the test works under
the soon-to-be-removed GenASM.

```
% qscalac -Ybackend:GenASM test/files/run/trait-static-forwarder/forwarders.scala && javac -d . -classpath . test/files/run/trait-static-forwarder/Test.java && qscala Test
./T.class: warning: Cannot find annotation method 'bytes()' in type 'ScalaSignature': class file for scala.reflect.ScalaSignature not found
1 warning
42
```

Fixes scala/scala-dev#59

Author: retronym

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

@@ -170,7 +170,7 @@ abstract class BCodeSkelBuilder extends BCodeHelpers {
 
       } else {
 
-        val skipStaticForwarders = (claszSymbol.isInterface || settings.noForwarders)
+        val skipStaticForwarders = settings.noForwarders
         if (!skipStaticForwarders) {
           val lmoc = claszSymbol.companionModule
           // add static forwarders if there are no name conflicts; see bugs #363 and #1735

src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala

@@ -1311,7 +1311,7 @@ abstract class GenASM extends SubComponent with BytecodeWriters { self =>
         for (constructor <- c.lookupStaticCtor) {
           addStaticInit(Some(constructor))
         }
-        val skipStaticForwarders = (c.symbol.isInterface || settings.noForwarders)
+        val skipStaticForwarders = settings.noForwarders
         if (!skipStaticForwarders) {
           val lmoc = c.symbol.companionModule
           // add static forwarders if there are no name conflicts; see bugs #363 and #1735

test/files/run/trait-static-forwarder.check

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

test/files/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());
+    }
+}

test/files/run/trait-static-forwarder/forwarders.scala

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