Add -D_LIBCPP_ENABLE_HARDENED_MODE=1

[david-a-wheeler]

We should probably add -D_LIBCPP_ENABLE_HARDENED_MODE=1. It helps for C++ when using libc++ and the LLVM compiler. See: https://libcxx.llvm.org/Hardening.html#using-hardening-modes

[thesamesam]

This bug is essentially a dupe of #149. LLVM keeps changing the option name. Please see #149 (comment) in particular.

[thomasnyman]

Thanks @thesamesam for keeping us updated on this. We jumped the gun a bit with the deprecation notes for the older macros as we were basing this on the proposed changes to LLVM 17. Given that these and the older alternatives have been unstable in the past, I would suggest we wait with further additions to the guide until the new hardened mode becomes available in a LLVM release, hopefully with LLVM 18.0.