Cleaned up generated code for closed Enums (apple#1751)

For closed enums there no special `UNRECOGNIZED` case, so a plain Swift
`Int` enum with explicit values can be used directly.

This results in the generated `init?(rawValue:)` and `var rawValue`
functions not being needed, since the compiler will create them
automatically.

Author: nicked

CompileTests/MultiModule/Sources/ModuleA/a.pb.swift

@@ -20,33 +20,15 @@ fileprivate struct _GeneratedWithProtocGenSwiftVersion: SwiftProtobuf.ProtobufAP
   typealias Version = _2
 }
 
-public enum E: SwiftProtobuf.Enum, Swift.CaseIterable {
-  public typealias RawValue = Int
-  case unset // = 0
-  case a // = 1
-  case b // = 2
+public enum E: Int, SwiftProtobuf.Enum, Swift.CaseIterable {
+  case unset = 0
+  case a = 1
+  case b = 2
 
   public init() {
     self = .unset
   }
 
-  public init?(rawValue: Int) {
-    switch rawValue {
-    case 0: self = .unset
-    case 1: self = .a
-    case 2: self = .b
-    default: return nil
-    }
-  }
-
-  public var rawValue: Int {
-    switch self {
-    case .unset: return 0
-    case .a: return 1
-    case .b: return 2
-    }
-  }
-
 }
 
 public struct A: SwiftProtobuf.ExtensibleMessage, Sendable {

FuzzTesting/Sources/FuzzCommon/fuzz_testing.pb.swift

@@ -35,36 +35,16 @@ fileprivate struct _GeneratedWithProtocGenSwiftVersion: SwiftProtobuf.ProtobufAP
   typealias Version = _2
 }
 
-public enum SwiftProtoTesting_Fuzz_AnEnum: SwiftProtobuf.Enum, Swift.CaseIterable {
-  public typealias RawValue = Int
-  case zero // = 0
-  case one // = 1
-  case two // = 2
-  case three // = 3
+public enum SwiftProtoTesting_Fuzz_AnEnum: Int, SwiftProtobuf.Enum, Swift.CaseIterable {
+  case zero = 0
+  case one = 1
+  case two = 2
+  case three = 3
 
   public init() {
     self = .zero
   }
 
-  public init?(rawValue: Int) {
-    switch rawValue {
-    case 0: self = .zero
-    case 1: self = .one
-    case 2: self = .two
-    case 3: self = .three
-    default: return nil
-    }
-  }
-
-  public var rawValue: Int {
-    switch self {
-    case .zero: return 0
-    case .one: return 1
-    case .two: return 2
-    case .three: return 3
-    }
-  }
-
 }
 
 public struct SwiftProtoTesting_Fuzz_Message: SwiftProtobuf.ExtensibleMessage, @unchecked Sendable {

Reference/CompileTests/MultiModule/Sources/ModuleA/a.pb.swift

@@ -20,33 +20,15 @@ fileprivate struct _GeneratedWithProtocGenSwiftVersion: SwiftProtobuf.ProtobufAP
   typealias Version = _2
 }
 
-public enum E: SwiftProtobuf.Enum, Swift.CaseIterable {
-  public typealias RawValue = Int
-  case unset // = 0
-  case a // = 1
-  case b // = 2
+public enum E: Int, SwiftProtobuf.Enum, Swift.CaseIterable {
+  case unset = 0
+  case a = 1
+  case b = 2
 
   public init() {
     self = .unset
   }
 
-  public init?(rawValue: Int) {
-    switch rawValue {
-    case 0: self = .unset
-    case 1: self = .a
-    case 2: self = .b
-    default: return nil
-    }
-  }
-
-  public var rawValue: Int {
-    switch self {
-    case .unset: return 0
-    case .a: return 1
-    case .b: return 2
-    }
-  }
-
 }
 
 public struct A: SwiftProtobuf.ExtensibleMessage, Sendable {

Reference/Conformance/editions/test_messages_proto2_editions.pb.swift

@@ -34,33 +34,15 @@ fileprivate struct _GeneratedWithProtocGenSwiftVersion: SwiftProtobuf.ProtobufAP
   typealias Version = _2
 }
 
-enum ProtobufTestMessages_Editions_Proto2_ForeignEnumProto2: SwiftProtobuf.Enum, Swift.CaseIterable {
-  typealias RawValue = Int
-  case foreignFoo // = 0
-  case foreignBar // = 1
-  case foreignBaz // = 2
+enum ProtobufTestMessages_Editions_Proto2_ForeignEnumProto2: Int, SwiftProtobuf.Enum, Swift.CaseIterable {
+  case foreignFoo = 0
+  case foreignBar = 1
+  case foreignBaz = 2
 
   init() {
     self = .foreignFoo
   }
 
-  init?(rawValue: Int) {
-    switch rawValue {
-    case 0: self = .foreignFoo
-    case 1: self = .foreignBar
-    case 2: self = .foreignBaz
-    default: return nil
-    }
-  }
-
-  var rawValue: Int {
-    switch self {
-    case .foreignFoo: return 0
-    case .foreignBar: return 1
-    case .foreignBaz: return 2
-    }
-  }
-
 }
 
 /// This proto includes every type of field in both singular and repeated
@@ -1033,38 +1015,18 @@ struct ProtobufTestMessages_Editions_Proto2_TestAllTypesProto2: SwiftProtobuf.Ex
 
   }
 
-  enum NestedEnum: SwiftProtobuf.Enum, Swift.CaseIterable {
-    typealias RawValue = Int
-    case foo // = 0
-    case bar // = 1
-    case baz // = 2
+  enum NestedEnum: Int, SwiftProtobuf.Enum, Swift.CaseIterable {
+    case foo = 0
+    case bar = 1
+    case baz = 2
 
     /// Intentionally negative.
-    case neg // = -1
+    case neg = -1
 
     init() {
       self = .foo
     }
 
-    init?(rawValue: Int) {
-      switch rawValue {
-      case -1: self = .neg
-      case 0: self = .foo
-      case 1: self = .bar
-      case 2: self = .baz
-      default: return nil
-      }
-    }
-
-    var rawValue: Int {
-      switch self {
-      case .neg: return -1
-      case .foo: return 0
-      case .bar: return 1
-      case .baz: return 2
-      }
-    }
-
   }
 
   struct NestedMessage: @unchecked Sendable {
@@ -1374,30 +1336,14 @@ struct ProtobufTestMessages_Editions_Proto2_EnumOnlyProto2: Sendable {
 
   var unknownFields = SwiftProtobuf.UnknownStorage()
 
-  enum BoolEnum: SwiftProtobuf.Enum, Swift.CaseIterable {
-    typealias RawValue = Int
-    case kFalse // = 0
-    case kTrue // = 1
+  enum BoolEnum: Int, SwiftProtobuf.Enum, Swift.CaseIterable {
+    case kFalse = 0
+    case kTrue = 1
 
     init() {
       self = .kFalse
     }
 
-    init?(rawValue: Int) {
-      switch rawValue {
-      case 0: self = .kFalse
-      case 1: self = .kTrue
-      default: return nil
-      }
-    }
-
-    var rawValue: Int {
-      switch self {
-      case .kFalse: return 0
-      case .kTrue: return 1
-      }
-    }
-
   }
 
   init() {}
@@ -1817,38 +1763,18 @@ struct ProtobufTestMessages_Editions_Proto2_TestAllRequiredTypesProto2: SwiftPro
 
   var unknownFields = SwiftProtobuf.UnknownStorage()
 
-  enum NestedEnum: SwiftProtobuf.Enum, Swift.CaseIterable {
-    typealias RawValue = Int
-    case foo // = 0
-    case bar // = 1
-    case baz // = 2
+  enum NestedEnum: Int, SwiftProtobuf.Enum, Swift.CaseIterable {
+    case foo = 0
+    case bar = 1
+    case baz = 2
 
     /// Intentionally negative.
-    case neg // = -1
+    case neg = -1
 
     init() {
       self = .foo
     }
 
-    init?(rawValue: Int) {
-      switch rawValue {
-      case -1: self = .neg
-      case 0: self = .foo
-      case 1: self = .bar
-      case 2: self = .baz
-      default: return nil
-      }
-    }
-
-    var rawValue: Int {
-      switch self {
-      case .neg: return -1
-      case .foo: return 0
-      case .bar: return 1
-      case .baz: return 2
-      }
-    }
-
   }
 
   struct NestedMessage: @unchecked Sendable {