fortran: Reuse associated_dummy memory if previously allocated [PR108923]

fortran: Reuse associated_dummy memory if previously allocated [PR108923]

This avoids making the associted_dummy field point to a new memory chunk
if it's already pointing somewhere, in which case doing so would leak the
previously allocated chunk.

	PR fortran/108923

gcc/fortran/ChangeLog:

	* intrinsic.cc (get_intrinsic_dummy_arg,
	set_intrinsic_dummy_arg): Rename the former to the latter.
	Remove the return value, add a reference to the lhs as argument,
	and do the pointer assignment inside the function.  Don't do
	it if the pointer is already non-NULL.
	(sort_actual): Update caller.

(cherry picked from commit 5c638095e7e0fa4de4e4f7326384a86830b25732)