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/ElseOnNullableEnum.kt

@@ -22,7 +22,7 @@ fun withNullableNothing(e: E?) = when (e) {
     nullableNothing() -> null
 }
 
-fun platformType() = <!NO_ELSE_IN_WHEN!>when<!> (J.foo()) {
+fun platformType() = when (<!WHEN_ENUM_CAN_BE_NULL_IN_JAVA!>J.foo()<!>) {
     E.A -> 7
     E.B -> 8
 }