Add to `isense_driver` the ability to output a preprocessed file

Opening this issue to track an internal work item. We should add the ability to isense_driver to preprocess a file.

To isolate user-reported issues, it's useful to preprocess the file (such as using the -E arg to gcc). That allows us to provide a repro to IntelliSense developers that does not depend on headers and defines from a particular environment. Unfortunately, compilers may not process the file exactly as IntelliSense does, resulting in preprocessed output that no longer reproduces the issue. Adding support to isense_driver to output preprocessed files would improve our ability to investigate those issues.