Auto merge of #148706 - dianne:slightly-lazier-temporary-scoping, r=cjgillot

compute temporary scopes when building MIR, not THIR

This accomplishes two things:
- Makes the THIR slightly smaller by not attaching a full `TempLifetime` to every expression.
- Reduces the number of traversals of the `ScopeTree` by only calling `ScopeTree::temporary_scope` when building MIR for something that needs to be dropped in a temporary scope.

Author: bors