For CUDA source, Sema checks that the targets of call expressions make sense (e.g. a host function can't call a device function).
Adding a flag that lets us skip this check. Motivation: for source-to-source translation tools that have to accept code that's not strictly kosher CUDA but is still accepted by nvcc. The source-to-source translation tool can then fix the code and leave calls that are semantically valid for the actual compilation stage.