[ASTGen] Sync PluginMessages.swift with the SwiftSyntax changes for extension

macros.

(cherry picked from commit c3bdae6)

Author: hborla

lib/ASTGen/Sources/ASTGen/Macros.swift

@@ -816,6 +816,14 @@ func expandAttachedMacroIPC(
     parentDeclSyntax = nil
   }
 
+  let extendedTypeSyntax: PluginMessage.Syntax?
+  if (!qualifiedType.isEmpty) {
+    let typeSyntax: TypeSyntax = "\(raw: qualifiedType)"
+    extendedTypeSyntax = .init(syntax: Syntax(typeSyntax), in: parentDeclSourceFilePtr!)!
+  } else {
+    extendedTypeSyntax = nil
+  }
+
   // Send the message.
   let message = HostToPluginMessage.expandAttachedMacro(
     macro: .init(moduleName: macro.moduleName, typeName: macro.typeName, name: macroName),
@@ -824,7 +832,8 @@ func expandAttachedMacroIPC(
     qualifiedType: qualifiedType,
     attributeSyntax: customAttributeSyntax,
     declSyntax: declSyntax,
-    parentDeclSyntax: parentDeclSyntax)
+    parentDeclSyntax: parentDeclSyntax,
+    extendedTypeSyntax: extendedTypeSyntax)
   do {
     let expandedSource: String?
     let diagnostics: [PluginMessage.Diagnostic]

lib/ASTGen/Sources/ASTGen/PluginMessages.swift

@@ -35,7 +35,8 @@ internal enum HostToPluginMessage: Codable {
     qualifiedType: String,
     attributeSyntax: PluginMessage.Syntax,
     declSyntax: PluginMessage.Syntax,
-    parentDeclSyntax: PluginMessage.Syntax?
+    parentDeclSyntax: PluginMessage.Syntax?,
+    extendedTypeSyntax: PluginMessage.Syntax?
   )
 
   /// Optionally implemented message to load a dynamic link library.
@@ -77,7 +78,7 @@ internal enum PluginToHostMessage: Codable {
 }
 
 /*namespace*/ internal enum PluginMessage {
-  static var PROTOCOL_VERSION_NUMBER: Int { 5 }  // Added 'expandMacroResult'.
+  static var PROTOCOL_VERSION_NUMBER: Int { 6 }  // Added 'expandMacroResult'.
 
   struct HostCapability: Codable {
     var protocolVersion: Int