[llvm] [Offload][UnitTests] Build device code as C++ (PR #151714)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

This looks fine to me, but I'll wait until one of the codeplay people chime in before merging.

https://github.com/llvm/llvm-project/pull/151714