PR fortran/40940
* decl.c (gfc_match_type_spec): Match CLASS statement and warn about
missing polymorphism.
* gfortran.h (gfc_typespec): Add field 'is_class'.
* misc.c (gfc_clear_ts): Initialize 'is_class' to zero.
* resolve.c (type_is_extensible): New function to check if a derived
type is extensible.
(resolve_fl_variable_derived): Add error checks for CLASS variables.
(resolve_typebound_procedure): Disallow non-polymorphic passed-object
dummy arguments, turning warning into error.
(resolve_fl_derived): Use 'type_is_extensible'. Disallow non-polymorphic
passed-object dummy arguments for procedure pointer components,
turning warning into error. Add error check for CLASS components.