[AST] Add new declaration - using

Initially this declaration is going to be used to determine
per-file default actor isolation i.e. `using @MainActor` and
`using nonisolated` but it could be extended to support other
file-global settings in the future.

(cherry picked from commit aabfebec03)

lib/Serialization/Serialization.cpp

@@ -2151,6 +2151,7 @@ static bool shouldSerializeMember(Decl *D) {
   case DeclKind::Extension:
   case DeclKind::Module:
   case DeclKind::PrecedenceGroup:
+  case DeclKind::Using:
     if (D->getASTContext().LangOpts.AllowModuleWithCompilerErrors)
       return false;
     llvm_unreachable("decl should never be a member");
@@ -5269,6 +5270,10 @@ public:
     llvm_unreachable("import decls should not be serialized");
   }
 
+  void visitUsingDecl(const UsingDecl *) {
+    llvm_unreachable("using decls should not be serialized");
+  }
+
   void visitEnumCaseDecl(const EnumCaseDecl *) {
     llvm_unreachable("enum case decls should not be serialized");
   }