Fortran: Fix memory leaks in PDT tests. [PR121972]

2026-06-22  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
	PR fortran/121972
	* decl.cc (build_struct): Convert the PDT template before the
	call to gfc_build_class_symbol.

gcc/testsuite/
	PR fortran/121972
	* gfortran.dg/pdt_77.f03: Add 'finalalize_navier_stokes', which
	tests that the 'navier_stokes' component 'p' is pointed to the
	correct vtable in both tests. Check that VIEW_CONVERT_EXPR does
	not appear in the tree dump.