[libc++] NFCI: Implement make_shared as allocate_shared with std::allocator

This simplifies the implementation, and it appears to be equivalent since
make_shared was allocating memory with std::allocator anyway.

Differential Revision: https://reviews.llvm.org/D93071

GitOrigin-RevId: ece3e5bb8b05370645b0add95c0da04f4a740724
3 files changed