[Patch, Fortran] PR54301 - add warning for pointer might outlive its target

[Tobias Burnus]

As suggested in the draft Fortran appendix to ISO/IEC Technical Report 
24772 "Guidance for Avoiding Vulnerabilities through Language Selection 
and Use"*

"Future standardization efforts should consider:"
...
"* Requiring that processors have the ability to detect and report the
occurrence within a submitted program unit of pointer assignment of
a pointer whose lifetime is known to be longer than the lifetime of the
target."

The new flag -Wtarget-lifetime implements this; it is enabled by -Wall. 
(Well, at least kind of; the suggested requirement is only implementable 
by a complicated run-time check. However, the attached compile-time 
warning is a good approximation, which should help to find real bugs 
with only few false positives.)

Build and regtested on x86-64-gnu-linux.
OK for the trunk?

Tobias

* See 
http://gcc.gnu.org/wiki/GFortranStandards#ISO.2BAC8-IEC_Project_22.24772:_Guidance_for_Avoiding_Vulnerabilities_through_Language_Selection_and_Use
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