Made bridge methods public.

Added test on usage of a bridge method from different module.
Bridge methods must be public in order use them from other modules.

backend.native/tests/codegen/bridges/linkTest_lib.kt

@@ -0,0 +1,9 @@
+package a
+
+interface A<T> {
+    fun foo(): T
+}
+
+open class C: A<Int> {
+    override fun foo(): Int = 42
+}

backend.native/tests/codegen/bridges/linkTest_main.kt

@@ -0,0 +1,12 @@
+import a.*
+
+class B: C()
+
+fun main(args: Array<String>) {
+    val b = B()
+    println(b.foo())
+    val c: C = b
+    println(c.foo())
+    val a: A<Int> = b
+    println(a.foo())
+}