Remove `PartialOrd`/`Ord` impls for `Span`/`SpanData`

`Span` and `SpanData` encode four fields: `lo`/`hi`, `ctxt`, and
`parent`. `ctxt` and `parent` have unorderable types.

Both types ignore `ctxt` and `parent` for `PartialOrd`/`Ord`. But their
`PartialEq`/`Eq` impls do *not* ignore those fields. This eq/ord
inconsistency is a bug. It was introduced in #123165.

The idea of ordering spans in general is dubious, because of `ctxt` and
`parent`. But the idea of ordering spans just with `lo`/`hi` is fine.
Therefore, this commit does the following.

- Removes the `PartialOrd`/`Ord` impls for `Span`/`SpanData`

- Adds a `Span::lo_hi` method which can be used in lots of places where
  span locations are involved in sorting. E.g.
  `xs.sort_by_key(|span| span.lo_hi())`

- Adds `OrdSpan`, a newtype around `Span` that impls
  `PartialEq`/`Eq`/`PartialOrd`/`Ord` using `lo_hi`. This is for storing
  spans in ordered types like `BTreeMap<OrdSpan, T>`.

Note also that some `sort`+`dedup` combinations might not remove all
duplicates with the old eq/ord inconsistency. These now all do the right
thing, which could affect some error messages, though in practice
nothing in the test suite is affected.