Mark a number of generated enums as frozen.

Long-term, this isn't what we want, but it avoids a miscompile in older
Swift versions (5.5).

Author: DougGregor

Sources/SwiftSyntax/Raw/RawSyntaxNodes.swift.gyb

@@ -39,6 +39,7 @@ public struct Raw${node.name}: Raw${node.name if node.is_base() else node.base_t
 %      enums.append(('Element', [(NODE_MAP[choice_name].swift_syntax_kind, NODE_MAP[choice_name].name) for choice_name in node.collection_element_choices]))
 %   end
 %   for (name, choices) in enums:
+  @frozen
   public enum ${name}: RawSyntaxNodeProtocol {
 %     for (swift_name, type_name) in choices:
     case `${swift_name}`(Raw${type_name})

Sources/SwiftSyntax/SyntaxCollections.swift.gyb

@@ -45,6 +45,7 @@ public extension SyntaxCollection {
 % end
 public struct ${node.name}: SyntaxCollection, SyntaxHashable {
 % if node.collection_element_choices:
+  @frozen
   public enum Element: SyntaxChildChoices {
 %   for choice_name in node.collection_element_choices:
 %     choice = NODE_MAP[choice_name]

Sources/SwiftSyntax/SyntaxEnum.swift.gyb

@@ -20,6 +20,7 @@
 //===----------------------------------------------------------------------===//
 
 /// Enum to exhaustively switch over all different syntax nodes.
+@frozen
 public enum SyntaxEnum {
   case unknown(UnknownSyntax)
   case token(TokenSyntax)

Sources/SwiftSyntax/SyntaxKind.swift.gyb

@@ -19,6 +19,7 @@
 //===----------------------------------------------------------------------===//
 
 /// Enumerates the known kinds of Syntax represented in the Syntax tree.
+@frozen
 public enum SyntaxKind {
   case token
   case unknown

Sources/SwiftSyntax/TokenKind.swift.gyb

@@ -18,6 +18,7 @@
 //===----------------------------------------------------------------------===//
 
 /// Enumerates the kinds of tokens in the Swift language.
+@frozen
 public enum TokenKind: Hashable {
   case eof
 % for token in SYNTAX_TOKENS:
@@ -143,6 +144,7 @@ extension TokenKind: Equatable {
 }
 
 /// Plain token kind value, without an associated `String` value.
+@frozen
 public enum RawTokenKind: Equatable, Hashable {
   case eof
 % for token in SYNTAX_TOKENS: