This patch reverts the following commits:
015cd317ea (add missing HIDE_FROM_ABI)
420a204d52 (add _LIBCPP_NO_CFI)
31eeba3f7c (add __uninitialized_buffer)
It also reverts a small part of b935ab8e74
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