[SYCL] Pre-process footer file when -include-footer is used.

[zahiraam]

With the new option -include-footer=file.h, the name file.h and its content need to be pulled in at the end of the pre-processed file. The actual pre-processed file should have this form:

header.h file.cpp content of file.cpp integration-footer.h content-of-footer.h

[zahiraam]

I am okay with the changes. I presume this has been tested along with @mdtoguchi's Driver changes offline.

We should wait before proceeding with this. Still working on the other issue found when not in pre-processing mode. A few things might change here when that other issued is fixed.