Fix ICE in build_function_decl [PR116292]

Fix ICE in build_function_decl [PR116292]

Fix ICE by getting the vtype only when a derived or class type is
prevent.  Also take care about the _len component for unlimited
polymorphics.

gcc/fortran/ChangeLog:

	PR fortran/116292

	* trans-intrinsic.cc (conv_intrinsic_move_alloc): Get the vtab
	only for derived types and classes and adjust _len for class
	types.

gcc/testsuite/ChangeLog:

	* gfortran.dg/move_alloc_19.f90: New test.