rustfmt `tests/mir-opt`.

The only non-obvious changes:
- `building/storage_live_dead_in_statics.rs` has a `#[rustfmt::skip]`
  attribute to avoid reformating a table of data.
- Two `.mir` files have slight changes involving line numbers.
- In `unusual_item_types.rs` an `EMIT_MIR` annotation is moved to
  outside a function, which is the usual spot, because `tidy` complains
  if such a comment is indented.

The commit also tweaks the comments in `rustfmt.toml`.