[llvm] Remember LLVM_ENABLE_LIBCXX setting in installed configuration (PR #134990)

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

I copied the flags we use downstream, not sure why they are the way they are TBH. Will look more into that.

On a separate note though, would these flags be doing anything during the `ninja` step? We do not do an intermediate `ninja install` or anything.

https://github.com/llvm/llvm-project/pull/134990