[fortran,patch] Give the main program its proper name

[FX Coudert]

*ping* (I know it's not been long, but since it' the week-end, some  
might have a bit a free time for this rather simple patch)

> Attached simple patch gives the symbol for the main program its proper
> name, if any (otherwise, MAIN__ is used), while its assembler name is
> set to MAIN__. This allows for better error messages from the
> middle-end (instead of "In function MAIN__", we get "In function bar"
> if the main program is bar [1]), and also for better debugging: we can
> now set breakpoints on the main program (still called bar in my
> example) by typing "break bar" as well as "break MAIN__".
>
> When this patch is in, I'll also submit a patch to the middle-end that
> emits DW_AT_calling_convention to identify the main program (that's
> our lowest-numbered PR, PR 10220) which can then use this information.
>
> Bootstrapped and regtested on x86_64-linux, OK to commit? (I regtested
> with Tobias' interface patch; without it, there is one failure, which
> lead to isolate the bug Tobias fixed.)
>
> FX
>
>
> PS: I also fixed or trimmed down a few comments along the way.
>
> [1] Yes, we should also make the GCC error mechanism print "In main
> program 'bar'", but that's for another day ;-)