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

[Will Hawkins via llvm-commits]

hawkinsw wrote:

> > @rnk Sorry to have to ask -- this will be the first PR that I will make as someone with committer rights. Now that you have approved, may I "Squash and merge"?
> 
> Yes, you can Squash and merge, that is the common way.

Thank you!

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