[libc++] Make _VSTD and alias for std

There is no practical difference between `_VSTD` and `std` so we should just remove `_VSTD`. This is the first step.

Reviewed By: ldionne, #libc

Spies: jeroen.dobbelaere, wmaxey, EricWF, lebedev.ri, __simt__, dim, mgrang, sstefan1, wenlei, smeenai, libcxx-commits, #libc_vendors

Differential Revision: https://reviews.llvm.org/D117811