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

[David Truby via llvm-commits]

DavidTruby wrote:

This works for me now, except for the extra warnings during the compiler-rt build. Could we change the `append("-stdlib=libc++ ...)` to something like `set(LLVM_ENABLE_LIBCXX_SUPPORTED ON CACHE BOOL "")`
and then use that in the runtimes that remove `-nostdinc++` (which might only be flang-rt?) to add the flag?

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