Added 'synthetic' flag for bridge method.

#KT-3155 fixed

compiler/testData/loadKotlin/fun/PropagateSubclassOfComparable.kt

@@ -0,0 +1,5 @@
+package test
+
+class PropagateSubclassOfComparable(): Comparable<PropagateSubclassOfComparable> {
+    override fun compareTo(other: PropagateSubclassOfComparable): Int = throw IllegalStateException()
+}

compiler/testData/loadKotlin/fun/PropagateSubclassOfComparable.txt

@@ -0,0 +1,6 @@
+namespace test
+
+internal final class test.PropagateSubclassOfComparable : jet.Comparable<test.PropagateSubclassOfComparable> {
+    public final /*constructor*/ fun <init>(): test.PropagateSubclassOfComparable
+    public open override /*1*/ fun compareTo(/*0*/ other: test.PropagateSubclassOfComparable): jet.Int
+}