[libc++] Revert __uninitialized_buffer changes

This patch reverts the following commits:

    015cd317 (add missing HIDE_FROM_ABI)
    420a204d (add _LIBCPP_NO_CFI)
    31eeba3f (add __uninitialized_buffer)

It also reverts a small part of b935ab8e
which is required to make the stable_partition.pass.cpp test pass on GCC.

Some issues were pointed out in https://reviews.llvm.org/D152208 and
in https://reviews.llvm.org/D154017, so I am reverting this patch
until we have time to weigh the various solutions and get consensus
on the design of the API.

Differential Revision: https://reviews.llvm.org/D154161