Fortran: improve error recovery for EXTENDS_TYPE_OF() [PR106121]

gcc/fortran/ChangeLog:

	PR fortran/106121
	* simplify.cc (gfc_simplify_extends_type_of): Do not attempt to
	simplify when one of the arguments is a CLASS variable that was
	not properly declared.

gcc/testsuite/ChangeLog:

	PR fortran/106121
	* gfortran.dg/extends_type_of_4.f90: New test.

Co-authored-by: Steven G. Kargl <kargl@gcc.gnu.org>