[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