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

[David Truby via llvm-commits]

DavidTruby wrote:

I think this is probably better than what I was doing in #139569 so we should stick with this.
I'm not sure what your reference to my comment about what the user probably wanted is meant to be attached to; is it in reference to the next line about `nostdinc++` or the previous line about `compiler-rt`? I'm not really sure how it relates to either.

All I'm saying about `compiler-rt` is that because this patch unconditionally adds `-stdlib=libc++` you get loads of warnings, where you didn't before, because compiler-rt doesn't link to a C++ library so the flag ends up unsed. Maybe it would be easier to have compiler-rt strip that flag, in which case it'd fix it for a standalone compiler-rt build too?

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