[libc++][hardening] Deprecate `_LIBCPP_ENABLE_ASSERTIONS`.

`_LIBCPP_ENABLE_ASSERTIONS` was used to enable the "safe" mode in
libc++. Libc++ now provides the hardened mode and the debug mode that
replace the safe mode.

For backward compatibility, enabling `_LIBCPP_ENABLE_ASSERTIONS` now
enables the hardened mode. Note that the hardened mode provides
a narrower set of checks than the previous "safe" mode (only
security-critical checks that are performant enough to be used in
production).

Differential Revision: https://reviews.llvm.org/D154997