[llvm] [libcxx] [libc++][hardening] Rework macros for enabling the hardening mode. (PR #70575)

[Louis Dionne via llvm-commits]

================
@@ -55,19 +55,14 @@ Improvements and New Features
 - ``std::ranges::count`` is now optimized for ``vector<bool>::iterator``, which
   can lead up to 350x performance improvements.
 
-- The library now provides a hardened mode under which common cases of library undefined behavior will be turned into
-  a reliable program termination. Vendors can configure whether the hardened mode is enabled by default with the
-  ``LIBCXX_HARDENING_MODE`` variable at CMake configuration time. Users can control whether the hardened mode is
-  enabled on a per translation unit basis using the ``-D_LIBCPP_ENABLE_HARDENED_MODE=1`` macro. See
-  :ref:`the hardening documentation <using-hardening-modes>` for more details.
-
-- The library now provides a debug mode which is a superset of the safe mode, additionally enabling more expensive
-  checks that are not suitable to be used in production. This replaces the legacy debug mode that was removed in this
-  release. Unlike the legacy debug mode, this doesn't affect the ABI and doesn't require locking. Vendors can configure
-  whether the debug mode is enabled by default with the ``LIBCXX_HARDENING_MODE`` variable at CMake configuration time.
-  Users can control whether the debug mode is enabled on a per translation unit basis using the
-  ``-D_LIBCPP_ENABLE_DEBUG_MODE=1`` macro. See :ref:`the hardening documentation <using-hardening-modes>` for more
-  details.
+- The library now provides several hardening modes under which common cases of library undefined behavior will be turned
+  into a reliable program termination. The fast hardening mode enables a set of security-critical checks with minimal
----------------
ldionne wrote:

```suggestion
  into a reliable program termination. The ``fast`` hardening mode enables a set of security-critical checks with minimal
```

Same for other modes?

https://github.com/llvm/llvm-project/pull/70575