Replace VEC (varobj_update_result) with std::vector

Replace VEC (varobj_update_result) with std::vector

This patch replaces makes varobj_update return an std::vector, and
updates the fallouts.

To make that easier, the varobj_update_result is c++ified a bit.  I
added a constructor and initialized its fields in-class.  The newobj
vector is also made an std::vector, so that it's automatically freed
when varobj_update_result is destroyed and handled correctly by the
default move constructor.  I disabled copy constructor and assignment
for that structure, because normally it never needs to be copied, only
moved.

As a result, the newobj parameter of update_dynamic_varobj_children must
be changed to an std::vector.  The patch converts the other vector
parameters of update_dynamic_varobj_children to std::vector.  It's not
strictly necessary to do it in the same patch, but I think it makes
sense to do it.

gdb/ChangeLog:

	* varobj.h (struct varobj_update_result): Add constructor, add
	move constructor, disable copy and assign, initialize fields.
	<newobj>: Change type to std::vector.
	(varobj_update): Return std::vector.
	* varobj.c (install_dynamic_child): Change VEC parameters to
	std::vector and adjust.
	(update_dynamic_varobj_children): Likewise.
	(varobj_update): Return std::vector and adjust.
	* mi/mi-cmd-var.c (varobj_update_one): Adjust to vector changes.