Fortran: fix variable definition context checks for SELECT TYPE [PR123253]

Fortran: fix variable definition context checks for SELECT TYPE [PR123253]

Commit r16-6300 introduced a regression when checking the variable
definition context of SELECT TYPE variables where the selector was not a
dummy argument as the scan for the association target was too shallow.
Scan through association lists for the ultimate selector.

	PR fortran/123253

gcc/fortran/ChangeLog:

	* expr.cc (gfc_check_vardef_context): Replace simple check by a
	scan through the association targets for a dummy argument.

gcc/testsuite/ChangeLog:

	* gfortran.dg/associate_76.f90: Extended testcase.
	* gfortran.dg/associate_77.f90: New test.