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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Your #139569 will fail some buildbots for the same reasons that #134990 was reverted for and would need the same code to check whether -stdlib=libc++ is actually working. It also passes LLVM_ENABLE_LIBCXX to compiler-rt in a bootstrapping builds and therefore will cause the very same warnings you don't like about this PR. IMHO it is an issue with compiler-rt, the same warning you would get if building compiler-rt in a standalone build with `LLVM_ENABLE_LIBCXX=ON`. In #139569 you were still arguing that 
> most likely what the user wanted when passing -DLLVM_ENABLE_LIBCXX".

The `-nostdinc++` in `CMAKE_REQUIRED_FLAGS` issue is already the second time I fell over it. See #139771.

Whether `LLVM_ENABLE_LIBCXX=ON` should fail if libc++ is unavailable is not my decision; the current implementation just emits a warning. That could be changed and is what I would also prefer, but it will also cause many additional build failures, including of some buildbots that add `LLVM_ENABLE_LIBCXX=ON` without having libc++ available on the system.

If you like I can withdraw this PR and we iterate in #139569. 


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