[libc++][hardening] Check that _LIBCPP_HARDENING_MODE_DEFAULT is defined (#80353)

If the `_LIBCPP_HARDENING_MODE_DEFAULT` macro is not defined,
`_LIBCPP_HARDENING_MODE` will be considered defined but fail the check
for a valid hardening mode, resulting in a slightly less understandable
error (that error is really meant more to prevent users from passing
incorrect values such as `0` or `1` directly rather than catching
configuration issues).

Author: var-const

libcxx/include/__config

@@ -325,6 +325,12 @@
 // clang-format on
 
 #  ifndef _LIBCPP_HARDENING_MODE
+
+#    ifndef _LIBCPP_HARDENING_MODE_DEFAULT
+#      error _LIBCPP_HARDENING_MODE_DEFAULT is not defined. This definition should be set at configuration time in the \
+`__config_site` header, please make sure your installation of libc++ is not broken.
+#    endif
+
 #    define _LIBCPP_HARDENING_MODE _LIBCPP_HARDENING_MODE_DEFAULT
 #  endif