The compiler-powered is_constructible that we have in gcc 8 is powerful
enough to give the right answer to an is_constructible question
that would be hard for a pure-library implementation to get right
in a well-formed fashion. This is just adding a test for it. Tested
on Linux-PPC64, OK for trunk?

2017-12-16  Ville Voutilainen  <ville.voutilainen@gmail.com>

    PR libstdc++/68430
    * testsuite/20_util/is_constructible/68430.cc: New.