gdb: use gdb::optional instead of passing a pointer to gdb::array_view

gdb: use gdb::optional instead of passing a pointer to gdb::array_view

Following on from the previous commit, this commit changes the API of
value_struct_elt to take gdb::optional<gdb::array_view<value *>>
instead of a pointer to the gdb::array_view.

This makes the optional nature of the array_view parameter explicit.

This commit is purely a refactoring commit, there should be no user
visible change after this commit.

I have deliberately kept this refactor separate from the previous two
commits as this is a more extensive change, and I'm not 100% sure that
using gdb::optional for the parameter type, instead of a pointer, is
going to be to everyone's taste.  If there's push back on this patch
then this one can be dropped from the series.

gdb/ChangeLog:

	* ada-lang.c (desc_bounds): Use '{}' instead of NULL to indicate
	an empty gdb::optional when calling value_struct_elt.
	(desc_data): Likewise.
	(desc_one_bound): Likewise.
	* eval.c (structop_base_operation::evaluate_funcall): Pass
	gdb::array_view, not a gdb::array_view* to value_struct_elt.
	(eval_op_structop_struct): Use '{}' instead of NULL to indicate
	an empty gdb::optional when calling value_struct_elt.
	(eval_op_structop_ptr): Likewise.
	* f-lang.c (fortran_structop_operation::evaluate): Likewise.
	* guile/scm-value.c (gdbscm_value_field): Likewise.
	* m2-lang.c (eval_op_m2_high): Likewise.
	(eval_op_m2_subscript): Likewise.
	* opencl-lang.c (opencl_structop_operation::evaluate): Likewise.
	* python/py-value.c (valpy_getitem): Likewise.
	* rust-lang.c (rust_val_print_str): Likewise.
	(rust_range): Likewise.
	(rust_subscript): Likewise.
	(eval_op_rust_structop): Likewise.
	(rust_aggregate_operation::evaluate): Likewise.
	* valarith.c (value_user_defined_op): Likewise.
	* valops.c (search_struct_method): Change parameter type, update
	function body accordingly, and update header comment.
	(value_struct_elt): Change parameter type, update function body
	accordingly.
	* value.h (value_struct_elt): Update declaration.