[libc++] Support arrays in make_shared and allocate_shared (P0674R1)

This patch implements P0674R1, i.e. support for arrays in std::make_shared
and std::allocate_shared.

Co-authored-by: Zoe Carver <z.zoelec2@gmail.com>

Differential Revision: https://reviews.llvm.org/D62641

GitOrigin-RevId: e27a122b3abc5000e99239f3df6e6d4e32b7dc81
21 files changed