Don't use `kw::Empty` in `hir::Lifetime::ident`.

`hir::Lifetime::ident` currently sometimes uses `kw::Empty` for elided
lifetimes and sometimes uses `kw::UnderscoreLifetime`, and the
distinction is used when creating some error suggestions, e.g. in
`Lifetime::suggestion` and `ImplicitLifetimeFinder::visit_ty`. I found
this *really* confusing, and it took me a while to understand what was
going on.

This commit replaces all uses of `kw::Empty` in `hir::Lifetime::ident`
with `kw::UnderscoreLifetime`. It adds a new field
`hir::Lifetime::is_path_anon` that mostly replaces the old
empty/underscore distinction and makes things much clearer.

Some other notable changes:

- Adds a big comment to `Lifetime` talking about permissable field
  values.

- Adds some assertions in `new_named_lifetime` about what ident values
  are permissible for the different `LifetimeRes` values.

- Adds a `Lifetime::new` constructor that does some checking to make
  sure the `is_elided` and `is_anonymous` states are valid.

- `add_static_impl_trait_suggestion` now looks at `Lifetime::res`
  instead of the ident when creating the suggestion. This is the one
  case where `is_path_anon` doesn't replace the old empty/underscore
  distinction.

- A couple of minor pretty-printing improvements.

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