Implementation of KT-6399: platform types under when are now counted as not nullable but warning is generated. #KT-6399 Fixed.

A set of tests. Warning about Java enum nullable argument under when in particular situations.

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

@@ -0,0 +1,22 @@
+// See KT-6399: exhaustive whens on platform enums
+
+// FILE: J.java
+
+public enum J {
+    A, B;
+
+    public static J create() {
+        return J.A;
+    }
+}
+
+// FILE: K.kt
+
+fun foo(): Int {
+    // When is exhaustive including null
+    return when (J.create()) {
+        J.A -> 1
+        J.B -> 2
+        null -> 0
+    }
+}