[Bug c++/68257] Reject empty abi_tag attribute on inline namespace

[sbergman at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68257

--- Comment #4 from Stephan Bergmann <sbergman at redhat dot com> ---
Created attachment 36973
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36973&action=edit
proposed documentation patch