[stdlib] Remove '@_frozen' from enums that shouldn't be frozen

Error codes, FloatingPointRoundingRule, Mirror.AncestorRepresentation,
Mirror.DisplayStyle, and PlaygroundQuickLook.

Author: jrose-apple

stdlib/public/Platform/MachError.swift

@@ -12,7 +12,6 @@
 
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
 /// Enumeration describing Mach error codes.
-@_frozen // FIXME(sil-serialize-all)		
 @objc
 public enum MachErrorCode : Int32 {
   case success                  = 0

stdlib/public/Platform/POSIXError.swift

@@ -15,7 +15,6 @@
 #if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
 
 /// Enumeration describing POSIX error codes.
-@_frozen // FIXME(sil-serialize-all)		
 @objc
 public enum POSIXErrorCode : Int32 {
   /// Operation not permitted.
@@ -269,7 +268,6 @@ public enum POSIXErrorCode : Int32 {
 #elseif os(Linux) || os(Android)
 
 /// Enumeration describing POSIX error codes.
-@_frozen // FIXME(sil-serialize-all)		
 public enum POSIXErrorCode : Int32 {
   /// Operation not permitted.
   case EPERM           = 1

stdlib/public/core/Codable.swift.gyb

@@ -1101,7 +1101,6 @@ public struct CodingUserInfoKey : RawRepresentable, Equatable, Hashable {
 //===----------------------------------------------------------------------===//
 
 /// An error that occurs during the encoding of a value.
-@_frozen // FIXME(sil-serialize-all)
 public enum EncodingError : Error {
   /// The context in which the error occurred.
   @_fixed_layout // FIXME(sil-serialize-all)
@@ -1190,7 +1189,6 @@ public enum EncodingError : Error {
 }
 
 /// An error that occurs during the decoding of a value.
-@_frozen // FIXME(sil-serialize-all)
 public enum DecodingError : Error {
   /// The context in which the error occurred.
   @_fixed_layout // FIXME(sil-serialize-all)

stdlib/public/core/FloatingPoint.swift.gyb

@@ -1267,7 +1267,6 @@ public enum FloatingPointClassification {
 }
 
 /// A rule for rounding a floating-point number.
-@_frozen // FIXME(sil-serialize-all)
 public enum FloatingPointRoundingRule {
   /// Round to the closest allowed value; if two values are equally close, the
   /// one with greater magnitude is chosen.

stdlib/public/core/Mirror.swift

@@ -69,7 +69,6 @@ public struct Mirror {
   /// its mirror represents ancestor classes by initializing the mirror
   /// with an `AncestorRepresentation`. This setting has no effect on mirrors
   /// reflecting value type instances.
-  @_frozen // FIXME(sil-serialize-all)
   public enum AncestorRepresentation {
 
     /// Generates a default mirror for all ancestor classes.
@@ -152,7 +151,6 @@ public struct Mirror {
   /// Playgrounds and the debugger will show a representation similar
   /// to the one used for instances of the kind indicated by the
   /// `DisplayStyle` case name when the mirror is used for display.
-  @_frozen // FIXME(sil-serialize-all)
   public enum DisplayStyle {
     case `struct`, `class`, `enum`, tuple, optional, collection
     case dictionary, `set`
@@ -503,7 +501,6 @@ extension Mirror {
 ///         // With Swift 4.0 and Swift 3.2 and earlier, use PlaygroundQuickLook
 ///         // and the CustomPlaygroundQuickLookable protocol.
 ///     #endif
-@_frozen // FIXME(sil-serialize-all)
 @available(*, deprecated, message: "PlaygroundQuickLook will be removed in a future Swift version. For customizing how types are presented in playgrounds, use CustomPlaygroundDisplayConvertible instead.")
 public enum PlaygroundQuickLook {
   /// Plain text.