[CompilerSources] Register using declaration

(cherry picked from commit 202907d)

Author: xedin

SwiftCompilerSources/Sources/AST/Declarations.swift

@@ -117,6 +117,8 @@ final public class TopLevelCodeDecl: Decl {}
 
 final public class ImportDecl: Decl {}
 
+final public class UsingDecl: Decl {}
+
 final public class PrecedenceGroupDecl: Decl {}
 
 final public class MissingDecl: Decl {}

SwiftCompilerSources/Sources/AST/Registration.swift

@@ -36,6 +36,7 @@ public func registerAST() {
   registerDecl(ExtensionDecl.self)
   registerDecl(TopLevelCodeDecl.self)
   registerDecl(ImportDecl.self)
+  registerDecl(UsingDecl.self)
   registerDecl(PrecedenceGroupDecl.self)
   registerDecl(MissingDecl.self)
   registerDecl(MissingMemberDecl.self)