This patch introduces `_LIBCPP_{BEGIN/END}_EXPLICIT_ABI_ANNOTATIONS` and
marks everything within an
`_LIBCPP_{BEGIN,END}_UNVERSIONED_NAMESPACE_STD` (and any derivatives
like `_LIBCPP_{BEGIN,END}_NAMESPACE_STD`) implicitly by default. This
allows us to drop `_LIBCPP_HIDE_FROM_ABI` in most of the code base,
except for functions which shouldn't be `_LIBCPP_HIDE_FROM_ABI`.
This patch doesn't remove any `_LIBCPP_HIDE_FROM_ABI`s, since we have
over 13k of them in the code base. Actually dropping them will happen
over some time to avoid too many merge conflicts.
2e2d90b986