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

[Louis Dionne via llvm-commits]

================
@@ -45,17 +45,26 @@ user code will be to have assertions disabled. As a user, you can consult your
 vendor to know which level of hardening is enabled by default.
 
 Furthermore, independently of any vendor-selected default, users can always
-control which level of hardening is enabled in their code by defining
-``_LIBCPP_ENABLE_HARDENED_MODE=0|1`` (or ``_LIBCPP_ENABLE_SAFE_MODE=0|1``, or
-``_LIBCPP_ENABLE_DEBUG_MODE=0|1``) before including any libc++ header (we
-recommend passing ``-D_LIBCPP_ENABLE_HARDENED_MODE=X``, etc. to the compiler).
+control which level of hardening is enabled in their code by defining the macro
+``_LIBCPP_HARDENING_MODE`` before including any libc++ headers (preferably by
+passing ``-D_LIBCPP_HARDENING_MODE=X`` to the compiler). The macro can be
+set to one of the following possible values:
+
+- ``_LIBCPP_HARDENING_MODE_UNCHECKED``;
+- ``_LIBCPP_HARDENING_MODE_HARDENED``;
+- ``_LIBCPP_HARDENING_MODE_DEBUG_LITE``;
+- ``_LIBCPP_HARDENING_MODE_DEBUG``.
+
+The exact numeric values of these macros are unspecified and you should not rely
+on them (e.g. expect the values to be sorted in any way).
----------------
ldionne wrote:

```suggestion
The exact numeric values of these macros are unspecified and users should not rely
on them (e.g. expect the values to be sorted in any way).
```

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