[Patch, fortran, RFC] Add warning for missing location information

[Thomas Koenig]

Hello world,

the attached patch runs through gfortran's AST to check for missing
location information.

If something is found, a warning is issued.  I chose not to issue an
error because missing location information does not make a program
invalid.  The patch would cause testsuite regressions for changes
which cause missing location information, which is a good thing because
we want to catch missing locations as soon as they are (potentially)
introduced.

The checking is turned off if CHECKING_P is not defined, so it should
not slow down release compilers.

I used DK_NOTE here, but that is open to discussion.  Maybe it would be
better to define another warning class for this?

Regression-tested "as is".  I also tested that a note is actually
issued by temporarily re-introducing a bug that has been fixed in
the meantime.

What do you think?  OK for trunk, or should something be done
differently?

Regards

	Thomas