[Patch] Add C++14 feature querying macro `cxx_digit_separators` describing support for "Single-Quotation-Mark as a Digit Separator"

[Michel Morin]

Richard Smith wrote:
> I wanted to wait until after the C++ committee meeting so the new version of
> SD-6 could get more officially accepted

I guessed that was the case :)

> I'm holding off on the cxx_digit_separators patch for now. It's much easier
> to add a feature test macro than to remove one, so I'd like to wait and
> see if there's a demand for feature names that match the SD-6 macros
> before adding more.

OK, no problem.
Thanks for adding new SD-6 macros!

Regards,
Michel