Warning generation on non-exhaustive whens over enums + a set of tests + fixed test #KT-6227 Fixed

compiler/testData/diagnostics/tests/when/NonExhaustiveWarningNull.kt

@@ -0,0 +1,14 @@
+// Base for KT-6227
+enum class X { A, B, C, D }
+
+fun foo(arg: X?): String {
+    var res = "XXX"
+    // Should we report something here? Probably not, null is not an enum entry
+    when (arg) {
+        X.A -> res = "A"
+        X.B -> res = "B"
+        X.C -> res = "C"
+        X.D -> res = "D"
+    }
+    return res
+}