[libc++] Implement P0487R1 - Fixing operator>>(basic_istream&, CharT*)

Summary:
Avoid buffer overflow by replacing the pointer interface with an array reference interface in C++2a.
Tentatively ready on Batavia2018.

 https://wg21.link/lwg2499
 https://wg21.link/p0487

Reviewers: mclow.lists, ldionne, EricWF

Reviewed By: ldionne

Subscribers: libcxx-commits, cfe-commits, christof

Differential Revision: https://reviews.llvm.org/D51268

llvm-svn: 347377