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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

It is a different error, previous was a linking error (libc++ symbol not found in libstdc++). Now it does not find the libc++ headers. Same with [lab.llvm.org/buildbot#/builders/10/builds/3180](https://lab.llvm.org/buildbot/#/builders/10/builds/3180). 

If the bootstrapping compiler (clang 19) found them, and they are installed in /usr, shouldn't stage1 clang find them as well?

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