`Diagnostic` has 40 methods that return `&mut Self` and could be
considered setters. Four of them have a `set_` prefix. This doesn't seem
necessary for a type that implements the builder pattern. This commit
removes the `set_` prefixes on those four methods.

It's not used, and doesn't quite fit the general pattern.

Also, `Diagnostic::downgrade_to_delayed_bug` doesn't need to return
`&mut Self` for the same reason.

- Move comments onto corresponding `Diagnostic` methods.
- Make formatting more consistent.

`create_almost_fatal` and `emit_almost_fatal` are always used instead.

These are misleading, because the mixture of `Level` and
`rustc_errors::Level` makes it look like there are two different types
involved.

It has a single call site.

Because it's redundant w.r.t. `Diagnostic::is_lint`, which is present
for every diagnostic level.

`struct_lint_level_impl` was the only place that set the `Error` field
to `true`, and it's also the only place that calls
`Diagnostic::is_lint()` to set the `is_lint` field.