[cfe] Change integer values that encode nullability attributes

This CL also completes the renaming of "neither" into "undetermined" by
renaming kNeither value in the nullability enum in
runtime/vm/kernel_binary.h into kUndetermined.

Change-Id: Ia266bf1b35577212b696fb4a093340ee8e986256
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121989
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Dmitry Stefantsov <dmitryas@google.com>

pkg/kernel/lib/ast.dart

@@ -5339,18 +5339,6 @@ class _PublicName extends Name {
 
 /// Represents nullability of a type.
 enum Nullability {
-  /// Nullable types are marked with the '?' modifier.
-  ///
-  /// Null, dynamic, and void are nullable by default.
-  nullable,
-
-  /// Non-nullable types are types that aren't marked with the '?' modifier.
-  ///
-  /// Note that Null, dynamic, and void that are nullable by default.  Note also
-  /// that some types denoted by a type parameter without the '?' modifier can
-  /// be something else rather than non-nullable.
-  nonNullable,
-
   /// Non-legacy types not known to be nullable or non-nullable statically.
   ///
   /// An example of such type is type T in the example below.  Note that both
@@ -5365,6 +5353,18 @@ enum Nullability {
   ///   }
   undetermined,
 
+  /// Nullable types are marked with the '?' modifier.
+  ///
+  /// Null, dynamic, and void are nullable by default.
+  nullable,
+
+  /// Non-nullable types are types that aren't marked with the '?' modifier.
+  ///
+  /// Note that Null, dynamic, and void that are nullable by default.  Note also
+  /// that some types denoted by a type parameter without the '?' modifier can
+  /// be something else rather than non-nullable.
+  nonNullable,
+
   /// Types in opt-out libraries are 'legacy' types.
   ///
   /// They are both subtypes and supertypes of the nullable and non-nullable

runtime/vm/kernel_binary.h

@@ -164,9 +164,9 @@ enum ConstantTag {
 
 // Keep in sync with package:kernel/lib/ast.dart
 enum Nullability {
-  kNullable = 0,
-  kNonNullable = 1,
-  kNeither = 2,
+  kUndetermined = 0,
+  kNullable = 1,
+  kNonNullable = 2,
   kLegacy = 3,
 };