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

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

@@ -0,0 +1,12 @@
+// Base for KT-6227
+enum class X { A, B, C, D }
+
+fun foo(arg: X): String {
+    val res: String
+    when (arg) {
+        X.A -> res = "A"
+        X.B -> res = "B"
+        else -> res = "CD"
+    }
+    return res
+}