Support -H and -P at the same time

The -P flag runs the dxc preprocessor.
The -H flag outputs a list of all headers included. (there is also a similar-Vi option)

The -H flag is only available when running a regular dxc build.
It is not available when running only the preprocessor.

This strikes me as odd, because as far as I can tell, the information that -H produces come from the preprocessor, not from the build. The preprocessor must know about all the includes to do its job.

I am personally in a situation where I only use dxc as a preprocessor, and I need to obtain the list of included headers. I have no way of doing so without running a full build, or manually parsing the preprocessed output.

The issue identifies the dxc -H and -P command-line options but names no source files or tests. Start by tracing how dxc handles these options, then verify that preprocessing alone can emit the included-header list when both flags are supplied.