auto merge of #11033 : michaelwoerister/rust/byvalself, r=pcwalton

As the title says. The trans changes will lead to an auxiliary alloca being created that allows debug info to track the `self` argument. This alloca is only created in debug builds however. Otherwise very little had to be done after I managed to navigate to some degree the jungle that is self-argument handling `:P`

Closes #10549