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/ExhaustivePlatformEnumStatement.kt

@@ -0,0 +1,20 @@
+// 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 (<!WHEN_ENUM_CAN_BE_NULL_IN_JAVA!>J.create()<!>) {
+        J.A -> return 1
+        J.B -> return 2
+    }
+}