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

[David Truby via llvm-commits]

DavidTruby wrote:

This doesn't fix the issue for me, on the same system that #139569 does. My cmake line is:
```
CC=clang CXX=clang++ cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_LIBCXX=On -DLLVM_ENABLE_PROJECTS="flang;clang" -DLLVM_ENABLE_RUNTIMES=openmp -DLLVM_TARGETS_TO_BUILD=host -DLLVM_CCACHE_BUILD=On -DLLVM_ENABLE_LLD=On ../llvm
```
and then `ninja && ninja check-flang-rt` fails with the same linker errors. I definitely have libc++ installed, but the tests in `check-flang-rt` are not adding -stdlib=libc++. 

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