Finish constification of varobj interface

Finish constification of varobj interface

This completes the constification of the struct varobj pointers in the
lang_varobj_ops interface partially done in
b09e2c591f9221d865bfe8425990a6bf9fab24e3. As suggested by Pedro,
varobj_get_path_expr casts away the const to assign the "mutable" struct
member.

gdb/ChangeLog:

	* ada-varobj.c (ada_name_of_child): Constify parent.
	(ada_path_expr_of_child): Same.
	(ada_value_of_child): Same.
	(ada_type_of_child): Same.
	* c-varobj.c (c_is_path_expr_parent): Same.
	(c_describe_child): Same.
	(c_name_of_child): Same.
	(c_value_of_child): Same.
	(c_type_of_child): Same.
	(cplus_number_of_children): Same.
	(cplus_describe_child): Constify var.
	(cplus_name_of_child): Constify parent.
	(cplus_value_of_child): Same.
	(cplus_type_of_child): Same.
	* jv-varobj.c (java_name_of_child): Same.
	(java_value_of_child): Same.
	(java_type_of_child): Same.
	* varobj.c (value_of_child): Same.
	(varobj_default_is_path_expr_parent): Constify var, parent and return
	value.
	(varobj_get_path_expr): Constify var, modify path_expr through
	mutable_var.
	(install_new_value): Constify parent.
	(value_of_child): Constify parent.
	* varobj.h (struct varobj): Constify parent.
	(struct lang_varobj_ops): Constify name_of_child, value_of_child and
	type_of_child.
	(varobj_get_path_expr): Constify var.
	(varobj_get_path_expr_parent): Constify var and return value.