[llvm] Remember LLVM_ENABLE_LIBCXX setting in installed configuration (PR #139712)

[David Truby via llvm-commits]

https://github.com/DavidTruby requested changes to this pull request.

Even with the comment I've added, which does make this work, this seems to cause a lot of extra warnings in the other runtimes builds for me. It looks like `-stdlib=libc++` is being passed there too, and is then giving the `not used at compile time` warning because `-nostdinc++` is also being passed.

I believe we only want to add `-stdlib=libc++` for runtimes that _don't_ pass `-nostdinc++`. 

https://github.com/llvm/llvm-project/pull/139712