Prioritized synthetic and not synthetic functions.

compiler/testData/diagnostics/tests/overload/SyntheticAndNotSynthetic.kt

@@ -0,0 +1,7 @@
+fun Runnable(f: () -> Unit): Runnable = object : Runnable {
+    public override fun run() {
+        f()
+    }
+}
+
+val x = Runnable {  }