This commit adds some new `LocalDecl` methods:
- `with_source_info`, a most general constructor.
- `new`, a variant of `with_source_info` which represents the most
common use case.
- `internal` a modifying method (like the already present `immutable`).
It removes some old `LocalDecl` methods:
- `new_internal` and `new_local`, because they're subsumed by the new
methods.
- `new_return_place`, because it was identical to `new_temp`.
Finally, it cleans up all the use sites.
afd8a4e7ec