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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Why doesn't it add `-stdlib=libc++`? What is the CMake output for CXX_COMPILER_SUPPORTS_STDLIB_CHRONO? What does the `CMakeConfigureLog.yaml` log say? Where is libc++ installed?

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