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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Can you clarify? 
1. Which c++ standard library (.a/.so/includes headers) do you intend to use with the LLVM_ENABLE_PROJECTS (llvm, clang, etc)?
2. Which c++ standard library (.a/.so/includes headers) do you intend to use with the LLVM_ENABLE_RUNTIMES (compiler-rt, libc, flang-rt etc)?

Note that using different c++ standard libraries for each of them is not a good idea: LLVM_ENABLE_RUNTIMES unittests are using a modified version of GTest that links with LLVMSupport and compiled with the LLVM_ENABLE_PROJECTS options.

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