[libc++][PSTL] Copy the headers into libc++

We decided to integrate the PSTL into our own headers and only share the backend impletementations. This is a first step in that direction, specifically it copies the PSTL headers into the libc++ structure.

Reviewed By: ldionne, #libc

Spies: rodgert, mikhail.ramalho, jplehr, bcain, h-vetinari, Mordante, rarutyun, var-const, sstefan1, pcwang-thead, libcxx-commits, arichardson, mgrang, miyuki

Differential Revision: https://reviews.llvm.org/D141779