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

[David Spickett via llvm-commits]

DavidSpickett wrote:

In our case, the libcxx is not system installed, it's part of the llvm release package we use. So stage 1 clang has no way of knowing where that is unless we tell it. Or enable libcxx in stage 1, but that feels like we're then going to mix 2 versions of libcxx.

I'll figure that out.

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