There is room for improvement on some of these, but something is better
than nothing.

We can just get the error level in the `match` and then use
`DiagnosticBuilder::new`. This then means a number of `DiagCtxt`
functions are no longer needed, because this was the one place that used
them.

Note: the commit changes the treatment of spans for `Expect`, which was
different to all the other cases, but this has no apparent effect.

There are a bunch of them about 400 lines down, which is weird and
annoying. This commit moves them up and puts them in a more sensible
order.

`IntoDiagnostic` defaults to `G = ErrorGuaranteed`. Take advantage of
this in one place that currently doesn't.

Its single use can be replaced with `Diagnostic::new_with_messages`.

Lots of vectors of messages called `message` or `msg`. This commit
pluralizes them.

Note that `emit_message_default` and `emit_messages_default` both
already existed, and both process a vector, so I renamed the former
`emit_messages_default_inner` because it's called by the latter.

They're always `None`.

It's only ever set to `None`.

It has three variants, but only `LabelKind::Label` is ever used. This
means `SingleLabelManySpans::kind` can also be removed.

Some of the `Level::` qualifiers in this file are omitted. This commit
removes the remainder.

There are quite a few hand-written `IntoDiagnostic` impls.

There are only three. It's simpler to make the type
`DiagnosticBuilder<'_, ()>` from the start, no matter the level, than to
change the guarantee later.

It's unused. And this means `DiagnosticBuilderInner` no longer needs to
be separate from `DiagnosticBuilder`.