KT-2498 Bridge method is not generated for method when generic parameters are substituted in superclass

#KT-2498 Fixed

generateBridgeIfNeeded() now works as follows: for a given method, find
which declared methods does this method override via breadth-first search.
For each such declared method, generate a bridge if and only if its
signature is different. Keep different methods' signatures in a set to
avoid duplicated bridge signatures.

compiler/testData/codegen/bridges/twoParentsWithTheSameMethodOneBridge.kt

@@ -0,0 +1,22 @@
+trait A<T> {
+    fun foo(t: T) = "A"
+}
+
+trait B<T> {
+    fun foo(t: T) = "B"
+}
+
+class Z : A<Int>, B<Int> {
+    override fun foo(t: Int) = "Z"
+}
+
+
+fun box(): String {
+    val z = Z()
+    return when {
+        z.foo(0)            != "Z" -> "Fail #1"
+        (z : A<Int>).foo(0) != "Z" -> "Fail #2"
+        (z : B<Int>).foo(0) != "Z" -> "Fail #3"
+        else -> "OK"
+    }
+}