Host/Device interfaces: integration header vs host compiler.

See these two threads: https://reviews.llvm.org/D99190?id=333780#inline-935611 and https://reviews.llvm.org/D99190?id=333780#inline-935593.

In nutshell, @Naghasan suggests we consider using a host compiler to emit the data from the integration header. OpenMP-offload uses such approach for binding offloaded code with the host application. I assume CUDA and HIP compilers are doing something similar. I was going to check if we can apply this approach to SYCL and gather all pros and cons for both approaches.

_Originally posted by @bader in https://github.com/intel/llvm/issues/3471#issuecomment-812347761_