Fortran: Prevent direct references to PDT instances [PR108663]

2026-02-15  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
	PR fortran/108663
	* 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/108663
	* gfortran.dg/pdt_15.f03: Modify tree dump test for new prefix.
	* gfortran.dg/pdt_71.f03: Ditto.
	* gfortran.dg/pdt_79.f03: Ditto.
	* gfortran.dg/pdt_84.f03: New test.