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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

@gulfemsavrun @Sharjeel-Khan 

You are using `LLVM_ENABLE_LIBCXX` in your CI configurations. If it stops working it seems you were actually using libstdc++ for building the runtimes. Why are you using `LLVM_ENABLE_LIBCXX=ON`?

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