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

[Lukáš Zaoral via llvm-commits]

lzaoral wrote:

@ekilmer Thanks for the PR but in the meantime I've stopped contributing to projects that depend on LLVM so I won't test it.

Moreover, unless this change is also backported to LLVM 14, projects will still need to use some workaround for the original issue.

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