JVM IR: Generate SAM wrapper fields as synthetic final

compiler/testData/codegen/bytecodeText/sam/samWrapperPrivateFinalSyntheticField.kt

@@ -0,0 +1,16 @@
+// FILE: J.java
+
+public class J {
+    public static void g(Runnable r) {
+        r.run();
+    }
+}
+
+// FILE: test.kt
+
+fun f() {
+    val r: () -> Unit = {}
+    J.g(r)
+}
+
+// 1 private final synthetic Lkotlin/jvm/functions/Function0; function