[libc++] Implement P0608R3 - A sane variant converting constructor

Summary:
Prefer user-defined conversions over narrowing conversions and conversions to bool.

References:
 http://wg21.link/p0608

Reviewers: EricWF, mpark, mclow.lists

Reviewed By: mclow.lists

Subscribers: zoecarver, ldionne, libcxx-commits, cfe-commits, christof

Differential Revision: https://reviews.llvm.org/D44865

llvm-svn: 363692