[clang] Add fixit for using declaration with a (qualified) namespace (#94762)

For `using std::literals`, we now output:

    error: using declaration cannot refer to a namespace
        4 |   using std::literals;
          |         ~~~~~^
    note: did you mean 'using namespace'?
        4 |   using std::literals;
          |         ^
          |         namespace

Previously, we didn't have the note.

This only fires for qualified namespaces. Just `using std;` doesn't
trigger this, since using declarations without cxx scope specifier are
rejected earlier. Making that work is an exercise for future selves :)