[llvm] [llvm][CMake] Respect LIBCXX_HARDENING_MODE on command-line (PR #68541)

[Will Hawkins via llvm-commits]

hawkinsw wrote:

I recently ran into a significant amount of trouble doing a bootstrap build with hardening properly enabled and I hope that this fix will help people in the future. I also plan on posting a PR to the https://libcxx.llvm.org/Hardening.html documentation to note how the different settings can interplay. 

I hope that I did everything correctly. Please let me know if there is anything I can do to improve this PR.

Thanks!

https://github.com/llvm/llvm-project/pull/68541