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

[Eric Kilmer via llvm-commits]

ekilmer wrote:

> Is there any reason to not use `check_cxx_source_compiles` unconditionally?

LLVM has C API bindings, so it's possible that a user would only enable the C language. Unconditionally using `check_cxx_source_compiles` would fail due to a missing C++ compiler.

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