Auto merge of #108692 - nnethercote:dont-call-temporary_scope-twice, r=cjgillot

Don't call `temporary_scope` twice.

`mirror_expr_inner` calls `temporary_scope`. It then calls `make_mirror_unadjusted` which makes an identical call to `temporary_scope`.

This commit changes the `mirror_expr_inner` to get the `temp_lifetime` out of the expression produced by `make_mirror_unadjusted`, similar to how it currently gets the type.

r? `@cjgillot`

Author: bors