Fortran: Prevent direct references to PDT instances [PR104650]

2025-11-26  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
	PR fortran/104650
	* decl.cc (gfc_get_pdt_instance): Use PDT_PREFIX and
	PDT_PREFIX_LEN.
	* gfortran.h : Define PDT_PREFIX and PDT_PREFIX_LEN. Note that
	PDT_PREFIX must have at least two upper case letters.
	* module.cc (read_module): Use PDT_PREFIX and PDT_PREFIX_LEN.
	* resolve.cc (resolve_typebound_procedure): Both pdt_template
	and pdt_type resolve_bindings_derived dummies should be tested
	for LEN type parameters being assumed.
	* symbol.cc (gfc_pdt_is_instance_of): Update preceding comment
	and use PDT_PREFIX_LEN.

gcc/testsuite
	PR fortran/104650
	* gfortran.dg/pdt_70.f03: New test.