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

[David Truby via llvm-commits]

https://github.com/DavidTruby approved this pull request.

Just to clarify, I'm happy with this PR now except that it introduces a bunch of warnings to my builds that weren't there before. This was true of my attempt as well, for what it's worth, I just hadn't clocked it at the time.

If you'd rather that gets fixed on the compiler-rt side I can post a fix to compiler-rt to strip `-stdlib=libc++` in there so the warnings go away. It might also be necessary for the other runtimes though. Probably all of them except flang-rt?

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