[patch] c++/65046 Use abi-tag to fix building with -Werror=abi-tag

[Jonathan Wakely]

This adds the abi_tag attribute in a few places it's missing, so that
-Werror=abi-tag builds will work after Jason's upcoming patches to
make functions inherit the tag from their return type.

For the __sso_string type I disable the warning, because the fact it
uses std::__cxx11::string internally is not visible to users.

There are still some missing tags in the Debug Mode string and list
which I'll fix asap.

Preparing this patch reminded me that we currently have two copies of
the Catalog_info and Catalogs code in the unnamed namespace in
config/locale/gnu/messages_members.cc, one using the old string and
one using the new. We should really alter the code to not use
std::string so that the catalogs can be shared by both versions of the
messages facets.

Tested x86_64-linux, as a standard build, and also with Jason's WIP
patches and --enable-cxx-flags=-Werror=abi-tag.

Committed to trunk.

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