K1 opt-in: fix message/behavior for future error with WasExperimental

compiler/testData/diagnostics/testsWithStdLib/experimental/duration.fir.kt

@@ -0,0 +1,14 @@
+// !API_VERSION: 1.5
+
+import kotlin.time.Duration
+
+@OptIn(kotlin.time.ExperimentalTime::class)
+data class Some(val duration: Duration = Duration.INFINITE)
+
+@OptIn(kotlin.time.ExperimentalTime::class)
+fun foo(duration: Duration = Duration.INFINITE) {}
+
+fun test() {
+    Some()
+    foo()
+}