[patch, fortran] Fix two issues with unused/unset variable warnings.

[Thomas Koenig]

Hello world,

what it says in the ChangeLog.

OK for trunk?

Best regards

	Thomas

Fix two issues with unused/unset variable warnings.

This patch fixes an ICE and adds correct diagnostics for reading/writing
variables via namelist, plus sets error variables for STOP as used
so incorrect warnings are avoided.

gcc/fortran/ChangeLog:

	PR fortran/126058
	PR fortran/126090
	* gfortran.h (gfc_dt): Add member nml_where.
	* io.cc (match_dt_element): Set nml_where.
	(gfc_resolve_dt): Mark variable in namelist to be read with
	correct locus and VALUE_READ. Mark expression in write namelist as
	used.
	(match_io): Set nml_where for data transfer.
	* resolve.cc: Mark code->expr1 as used for PAUSE, STOP and ERROR
	STOP.

gcc/testsuite/ChangeLog:

	PR fortran/126058
	PR fortran/126090
	* gfortran.dg/warn_undefined_vars_5.f90: New test.
	* gfortran.dg/warn_undefined_vars_6.f90: New test.
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