glsl/glcpp: Add a -d/--debug option to the standalone glcpp program

glsl/glcpp: Add a -d/--debug option to the standalone glcpp program

The verbose debug output from the parser is quite useful when debugging, and
having this available as a command-line option is much more convenient than
manually forcing this into the code when needed, (which is what I had been
doing for too long previously).

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>