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

[David Truby via llvm-commits]

DavidTruby wrote:

My issue here is separate to the script; I'm just trying to get a simple case working outside of the script.
I don't appear to get any output for `CXX_COMPILER_SUPPORTS_STDLIB_CHRONO`. All I do see is: 
```
CMake Warning at .../llvm-project/llvm/cmake/modules/HandleLLVMStdlib.cmake:33 (message):
  Can't specify libc++ with '-stdlib='
Call Stack (most recent call first):
  .../llvm-project/llvm/cmake/modules/HandleLLVMOptions.cmake:11 (include)
  CMakeLists.txt:175 (include)
  ```

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