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

[Eric Kilmer via llvm-commits]

ekilmer wrote:

@JDevlieghere Great! I don't have commit access, so would you (or someone else we can ping) be able to help me out with landing this?

Thank you for the review!

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