RFA: Match calls to debug_end_source_file with calls to debug_start_source_file

[Neil Booth]

Richard Henderson wrote:-

> I'd rather thought Neil was going to approve this, since it's
> logically part of the preprocessor.  But anyway...

I did, but thought I'd better wait for a 2nd opinion. <g>

Cool, from now on I will condsider changes to the frontends' cpplib
interface part of CPP.  Anything I'm not sure of I'll ask anyway.

Hans-Peter, do you have write access?  If not, let me know.  Otherwise
I'll assume you will commit it.

Neil.