[fortran,patch] Fix PR 33739, emitting calls to debug_hooks->end_source_file

[FX Coudert <fxcoudert at gmail dot com>]

Attached patch fixes PR 33739, a debug-info problem which is only  
seen with stabs and the darwin 64-bit linker, but is nonetheless an  
issue in the front-end. Each time we enter an included source file,  
we emit a call to debug_hooks->start_source_file; we call debug_hooks- 
>end_source_file when returning to the original file; my recent  
patch to scanner.c made sure of that.

Unfortunately, there is one situation that was not yet addressed.  
When the inclusion line is the last line of the outer file, like this:

> $ cat b.F
>   print *, 42
> #include "b.inc"
> $ cat b.inc
>   end

In that case, the problem is that when we come into gfc_advance_line,  
it's too late; and during parsing, it's too early to emit calls to  
the debug_hooks functions. But it happens to be simple: I added  
functions to keep track of files entered and exited, so be able to  
emit exit calls for each remaining file when we reach the end of the  
main source file.

Regtested on x86_64-linux with dwarf and stabs, and regtested on  
powerpc-apple-darwin8 (stabs) with -m32 and -m64. OK to commit?

FX

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