[RFA] Remove make_cleanup_unpush_target

[Simon Marchi]

On 2018-03-29 11:22, Tom Tromey wrote:
> This removes make_cleanup_unpush_target, replacing it with a
> unique_ptr.  This may seem odd, because the object in question is not
> actually freed, but unique_ptr provided the necessary functionality.
> 
> Tested by the buildbot.

I was not sure about the usage of unique_ptr at first, but I guess it 
works just as well as writing our own object.  So it LGTM.

Simon