Method setInstance

include/ast/component/declarator.h:209–211

    return *std::get_if<InstanceDecl>(&Decl);
  }
  void setInstance(InstanceDecl &&Inst) noexcept {
    Decl.emplace<InstanceDecl>(std::move(Inst));
  }

  bool isImportDecl() const noexcept {
    return std::holds_alternative<ImportDecl>(Decl);