[CUDA] Warn about unsupported CUDA SDK version only if it's used.
    
    This fixes an issue with clang issuing a warning about unknown CUDA SDK if it's
    detected during non-CUDA compilation.
    
    Differential Revision: https://reviews.llvm.org/D76030
    
    (cherry picked from commit eb2ba2ea953b5ea73cdbb598f77470bde1c6a011)