K2: add a black box test to confirm KT-55705 now works properly

compiler/testData/codegen/box/smartCasts/genericInheritanceWithNonGenericNoAmbiguity.kt

@@ -0,0 +1,25 @@
+// ISSUE: KT-55705
+
+interface A<T> {
+    fun foo(x: T?) {}
+}
+
+interface B : A<String> {
+    override fun foo(x: String?)
+}
+
+fun <T> bar(x: A<in T>) {
+    if (x is B) {
+        // The code should be green
+        x.foo(null)
+    }
+}
+
+fun box(): String {
+    bar<String>(
+        object : B {
+            override fun foo(x: String?) {}
+        }
+    )
+    return "OK"
+}