RFA: Match calls to debug_end_source_file with calls to debug_start_source_file

[Jason Merrill]

While this patch definitely improves the situation, IMO it misses the
real bug: cb_enter_file works differently for includes of other
physical files and logical includes in a .i file.  It seems to me that
this distinction should be invisible to c-lex.c; if the preprocessor
uses a single buffer for the .i file, c-lex.c should not use
CPP_BUFFER at all.

Thoughts?

Jason