[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/cast/DowncastMap.fir.kt

@@ -0,0 +1,10 @@
+interface Map<K, out V>
+interface MutableMap<K, V>: Map<K, V> {
+  operator fun set(k: K, v: V)
+}
+
+fun p(p: Map<String, Int>) {
+    if (p is MutableMap<String, Int>) {
+        p[""] = 1
+    }
+}