Fix safe-init error in DesugarEnums

When bootstrapping the compiler with the `-Ysafe-init` flag, we would
get the following error:
```
[error] -- Error: /Users/********/dotty/compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala:20:39
[error] 20 |    val Simple, Object, Class: Value = Value
[error]    |                                       ^^^^^
[error]    |Calling the external method method Value may cause initialization errors.
```

This error orginates from the Scala 2 `Enumeration` class definition. In order to circumvent this issue, we instead define `CaseKind` using a Scala 3 enum. While this does require enum comparisons using `<` or `<=` to explicitly compare the `ordinal` of the enum variant, it also eliminates the initialization error.

Review by @liufengyun

Author: Xavientois

compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala

@@ -16,21 +16,20 @@ import scala.annotation.internal.sharable
 object DesugarEnums {
   import untpd._
 
-  @sharable object CaseKind extends Enumeration {
-    val Simple, Object, Class: Value = Value
-  }
+  enum CaseKind:
+    case Simple, Object, Class
 
-  final case class EnumConstraints(minKind: CaseKind.Value, maxKind: CaseKind.Value, enumCases: List[(Int, RefTree)]):
-    require(minKind <= maxKind && !(cached && enumCases.isEmpty))
+  final case class EnumConstraints(minKind: CaseKind, maxKind: CaseKind, enumCases: List[(Int, RefTree)]):
+    require(minKind.ordinal <= maxKind.ordinal && !(cached && enumCases.isEmpty))
     def requiresCreator = minKind == CaseKind.Simple
-    def isEnumeration   = maxKind < CaseKind.Class
-    def cached          = minKind < CaseKind.Class
+    def isEnumeration   = maxKind.ordinal < CaseKind.Class.ordinal
+    def cached          = minKind.ordinal < CaseKind.Class.ordinal
   end EnumConstraints
 
   /** Attachment containing the number of enum cases, the smallest kind that was seen so far,
    *  and a list of all the value cases with their ordinals.
    */
-  val EnumCaseCount: Property.Key[(Int, CaseKind.Value, CaseKind.Value, List[(Int, TermName)])] = Property.Key()
+  val EnumCaseCount: Property.Key[(Int, CaseKind, CaseKind, List[(Int, TermName)])] = Property.Key()
 
   /** Attachment signalling that when this definition is desugared, it should add any additional
    *  lookup methods for enums.
@@ -249,11 +248,11 @@ object DesugarEnums {
    *   - scaffolding containing the necessary definitions for singleton enum cases
    *     unless that scaffolding was already generated by a previous call to `nextEnumKind`.
    */
-  def nextOrdinal(name: Name, kind: CaseKind.Value, definesLookups: Boolean)(using Context): (Int, List[Tree]) = {
+  def nextOrdinal(name: Name, kind: CaseKind, definesLookups: Boolean)(using Context): (Int, List[Tree]) = {
     val (ordinal, seenMinKind, seenMaxKind, seenCases) =
       ctx.tree.removeAttachment(EnumCaseCount).getOrElse((0, CaseKind.Class, CaseKind.Simple, Nil))
-    val minKind = if kind < seenMinKind then kind else seenMinKind
-    val maxKind = if kind > seenMaxKind then kind else seenMaxKind
+    val minKind = if kind.ordinal < seenMinKind.ordinal then kind else seenMinKind
+    val maxKind = if kind.ordinal > seenMaxKind.ordinal then kind else seenMaxKind
     val cases = name match
       case name: TermName => (ordinal, name) :: seenCases
       case _              => seenCases