[llvm] [llvm][CMake] Check dependency cxx source compiles (PR #68549)

[Jonas Devlieghere via llvm-commits]

https://github.com/JDevlieghere approved this pull request.

Thanks, it wasn't clear to me that this was functionality was also triggered when including llvm from a downstream project. Thank you for clarifying that and even including an example. This shows that it's indeed necessary to check both `check_c_source_compiles` and `check_cxx_source_compiles`. LGTM!

https://github.com/llvm/llvm-project/pull/68549