fortran: Set name for *LOC default BACK argument [PR108450]

fortran: Set name for *LOC default BACK argument [PR108450]

This change fixes an ICE caused by the double resolution of MINLOC,
MAXLOC and FINDLOC expressions which get a default value for the BACK
argument at resolution time.  That argument  is added without name,
and argument reordering code is not prepared to handle unnamed arguments
coming after named ones, so the second resolution causes a NULL pointer
dereference.
The problem is fixed by explicitly setting the argument name.

	PR fortran/108450

gcc/fortran/ChangeLog:

	* check.cc (gfc_check_minloc_maxloc): Explicitly set argument name.
	(gfc_check_findloc): Ditto.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/minmaxloc_1.f90: New test.