flambda-backend: Implement the nullability axis for jkinds (#2731)

* Minimal

* tests

* Default more types to non-null

* format

* Creation reasons

* Comments

* Tests

* CR

* Update variables test

* Comments

* Consistent naming

* Update ocaml/testsuite/tests/typing-layouts-or-null/variables.ml

Co-authored-by: Richard Eisenberg <[email protected]>

* Tests

* More tests

* Remove redundant tests

* fix

* Rename to `concrete_default_creation_reason`

* Refactor mode crossing

* `Maybe_null`

* Update tests

* Format

---------

Co-authored-by: Diana Kalinichenko <[email protected]>
Co-authored-by: Richard Eisenberg <[email protected]>

Author: 3 people

testsuite/tests/typing-layouts-or-null/array.ml

@@ -0,0 +1,24 @@
+(* TEST
+ flags = "-extension layouts_alpha";
+ expect;
+*)
+
+(* CR layouts v3.0: array type arguments should be [any_non_null]: *)
+
+type t_any : any
+
+type should_fail = t_any array
+
+[%%expect{|
+type t_any : any
+type should_fail = t_any array
+|}]
+
+type t_value_or_null : value_or_null
+
+type should_fail = t_value_or_null array
+
+[%%expect{|
+type t_value_or_null : value_or_null
+type should_fail = t_value_or_null array
+|}]