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

[via llvm-commits]

AdvenamTacet wrote:

Hey, LGTM but let's wait for a review from someone who worked on those cmakes. @jackoalan @JDevlieghere as you implemented/reviewed [D114327](https://reviews.llvm.org/D114327), could you review that PR?

Also, @lzaoral does it fix your issue from https://github.com/llvm/llvm-project/issues/53950?

@ekilmer did you test it only on git-main version of LLVM?

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