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

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

If you need help from my end for the failing AMDGPU buildbot, let me know.
Happy to help.

If this is not an easy fix, can we revert in the meantime?

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