[C++14] Add is_final type trait.

[Jonathan Wakely]

On 13/09/14 21:58 -0400, Ed Smith-Rowland wrote:
>We've had __has_final built-in for a good while.
>the std library component is_final was added to C++14 - which is now good.
>I noticed while looking at the latest SD-6 draft.
>
>So here is a simple patch that builds and passes clean on x86_64-linux.
>
>OK?

Yes, OK for trunk. Thanks, Ed.

I'd be willing to approve a backport to 4.9 as well (changing the test
from >= 201402L to > 201103L) if you wanted to prepare that patch :-)