[v3] libstdc++/18571

[Benjamin Kosnik]

> Thanks. Therefore, I understand that later parts will deal with the 
> remnants of
> _GLIBCXX_USE___ENC_TRAITS in the testsuite? (just noticed, by chance)

Yes.

I'm not quite done with this stuff yet. I'm not exactly sure how to do
the testsuite bits, but am leaning more towards having this
functionality tested with an explicit switch, like threads of wchar_t.
So, your patience is appreciated while I work out the details.

-benjamin