Implement equality operator synthesization

Sources/FrontEnd/AST/AST.swift

@@ -303,7 +303,6 @@ public struct AST {
   /// Returns the kind identifying synthesized declarations of `requirement`, or `nil` if
   /// `requirement` is not synthesizable.
   public func synthesizedKind<T: DeclID>(of requirement: T) -> SynthesizedFunctionDecl.Kind? {
-    // If the requirement is defined in `Deinitializable`, it must be the deinitialization method.
     switch requirement.rawValue {
     case core.deinitializable.deinitialize.rawValue:
       return .deinitialize
@@ -313,6 +312,8 @@ public struct AST {
       return .moveAssignment
     case core.copyable.copy.rawValue:
       return .copy
+    case core.equatable.equal.rawValue:
+      return .equal
     default:
       return nil
     }

Sources/FrontEnd/AST/Decl/SynthesizedFunctionDecl.swift

@@ -18,6 +18,9 @@ public struct SynthesizedFunctionDecl: Hashable {
     /// A copy method.
     case copy
 
+    /// An equality method.
+    case equal
+
     /// A global initializer for a binding declaration.
     case globalInitialization(BindingDecl.ID)
 

Sources/FrontEnd/Types/BuiltinType.swift

@@ -28,6 +28,9 @@ public enum BuiltinType: TypeProtocol {
   /// The type of the built-in module.
   case module
 
+  /// The type of a union discriminator.
+  public static let discriminator = word
+
   /// `true` iff `self` is `.i` or `.word`.
   public var isInteger: Bool {
     switch self {