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

[David Spickett via llvm-commits]

DavidSpickett wrote:

I'm not going to be able to work on this due to travel, so I've written up what I know in https://github.com/llvm/llvm-project/issues/135381 and will get a colleague to pick it up. Maybe some of those notes will be useful for the other bots.

I have moved our bot to staging (https://lab.llvm.org/staging/#/builders/46), so if you do find fixes for the other bots and want to reland this, go ahead.

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