[JVM_IR] Follow old backend in bridge visibility and varargs marking.

The old backend makes bridges for protected and package-private
methods public. Also, for bridges for vararg methods, the vararg
marker is not on the bridge.

These differences seem minor but are visible via reflection, so
we might as well follow the old backend.

compiler/testData/codegen/bytecodeListing/varargsBridge.kt

@@ -0,0 +1,8 @@
+abstract class A<T> {
+    protected abstract fun doIt(vararg args: T): String
+    fun test() = doIt()
+}
+
+class B : A<Void>() {
+    override fun doIt(vararg args: Void): String = "OK"
+}