Make FE1.0/FIR OptIn override message more clear

compiler/testData/cli/jvm/optInOverrideMessage.kt

@@ -0,0 +1,19 @@
+@RequiresOptIn(message = "This API is experimental and can change at any time, please use with care")
+annotation class Marker
+
+@RequiresOptIn
+annotation class EmptyMarker
+
+interface Base {
+    @Marker
+    fun foo()
+
+    @EmptyMarker
+    fun bar()
+}
+
+class Derived : Base {
+    override fun foo() {}
+
+    override fun bar() {}
+}