Constify some parameters in the varobj code

Constify some parameters in the varobj code

To make it clear that some functions should not modify the variable
object, this patch adds the const qualifier where it makes sense to some
struct varobj * parameters. Most getters should take a const pointer to
guarantee they don't modify the object.

Unfortunately, I couldn't add it to some callbacks (such as name_of_child).
In the C implementation, they call c_describe_child, which calls
varobj_get_path_expr. varobj_get_path_expr needs to modify the object in
order to cache the computed value. It therefore can't take a const
pointer, and it affects the whole call chain. I suppose that's where you
would use a "mutable" in C++.

I did that to make sure there was no other cases like the one fixed in
the previous patch. I don't think it can hurt.

gdb/ChangeLog:

	* ada-varobj.c (ada_number_of_children): Constify struct varobj *
	parameter.
	(ada_name_of_variable): Same.
	(ada_path_expr_of_child): Same.
	(ada_value_of_variable): Same.
	(ada_value_is_changeable_p): Same.
	(ada_value_has_mutated): Same.
	* c-varobj.c (varobj_is_anonymous_child): Same.
	(c_is_path_expr_parent): Same.
	(c_number_of_children): Same.
	(c_name_of_variable): Same.
	(c_path_expr_of_child): Same.
	(get_type): Same.
	(c_value_of_variable): Same.
	(cplus_number_of_children): Same.
	(cplus_name_of_variable): Same.
	(cplus_path_expr_of_child): Same.
	(cplus_value_of_variable): Same.
	* jv-varobj.c (java_number_of_children): Same.
	(java_name_of_variable): Same.
	(java_path_expr_of_child): Same.
	(java_value_of_variable): Same.
	* varobj.c (number_of_children): Same.
	(name_of_variable): Same.
	(is_root_p): Same.
	(varobj_ensure_python_env): Same.
	(varobj_get_objname): Same.
	(varobj_get_expression): Same.
	(varobj_get_display_format): Same.
	(varobj_get_display_hint): Same.
	(varobj_has_more): Same.
	(varobj_get_thread_id): Same.
	(varobj_get_frozen): Same.
	(dynamic_varobj_has_child_method): Same.
	(varobj_get_gdb_type): Same.
	(is_path_expr_parent): Same.
	(varobj_default_is_path_expr_parent): Same.
	(varobj_get_language): Same.
	(varobj_get_attributes): Same.
	(varobj_is_dynamic_p): Same.
	(varobj_get_child_range): Same.
	(varobj_value_has_mutated): Same.
	(varobj_get_value_type): Same.
	(number_of_children): Same.
	(name_of_variable): Same.
	(check_scope): Same.
	(varobj_editable_p): Same.
	(varobj_value_is_changeable_p): Same.
	(varobj_floating_p): Same.
	(varobj_default_value_is_changeable_p): Same.
	* varobj.h (struct lang_varobj_ops): Consitfy some struct varobj *
	parameters.
	(varobj_get_objname): Constify struct varobj * parameter.
	(varobj_get_expression): Same.
	(varobj_get_thread_id): Same.
	(varobj_get_frozen): Same.
	(varobj_get_child_range): Same.
	(varobj_get_display_hint): Same.
	(varobj_get_gdb_type): Same.
	(varobj_get_language): Same.
	(varobj_get_attributes): Same.
	(varobj_editable_p): Same.
	(varobj_floating_p): Same.
	(varobj_has_more): Same.
	(varobj_is_dynamic_p): Same.
	(varobj_ensure_python_env): Same.
	(varobj_default_value_is_changeable_p): Same.
	(varobj_value_is_changeable_p): Same.
	(varobj_get_value_type): Same.
	(varobj_is_anonymous_child): Same.
	(varobj_value_get_print_value): Same.
	(varobj_default_is_path_expr_parent): Same.