trans::glue: don't allocate a pointer variable if it already exists

Removes one alloca and store from the drop glue of @ boxes. This speeds
up the rustc build by 1s (might be noise, though).
auto merge of #7155 : Blei/rust/drop-glue-alloca, r=graydon

Removes one alloca and store from the drop glue of @ boxes. This speeds
up the rustc build by 1s (might be noise, though).