Add position information to Sep

I'm writing a documentation generator based on cc: gyosu.

One limitation I'm running into is that position information is missing from Sep. This means I can't figure out where a documentation comment originates from. Given this file:

/**
 * This file contains an API to do a lot of useful things.
 */

#include <stdint.h>

int32_t foo(void);

I can extract the comment by looking at the first declaration, but I also get various comments originating from stdint.h itself.

It would be helpful to be able to figure out which part of the Sep comes from my own header, and which part comes from an #include'ed file.