KT-31908 Handle SAM conversion on vararg elements

compiler/testData/codegen/box/sam/kt31908.kt

@@ -0,0 +1,17 @@
+// TARGET_BACKEND: JVM
+// FILE: kt31908.kt
+fun box(): String {
+    var result = "failed"
+    val r = java.lang.Runnable { result += "K" }
+    J().foo({ result = "O" }, r)
+    return result
+}
+
+// FILE: J.java
+public class J {
+    public void foo(Runnable... rs) {
+        for (Runnable r : rs) {
+            r.run();
+        }
+    }
+}