`resolve_ident_in_lexical_scope` checks for an empty name. Why is this
necessary? Because `parse_item_impl` can produce an `impl` block with an
empty trait name in some cases. This is pretty gross and very
non-obvious.
This commit avoids the use of the empty trait name. In one case the
trait name is instead pulled from `TyKind::ImplTrait`, which prevents
the output for `tests/ui/impl-trait/extra-impl-in-trait-impl.rs` from
changing. In the other case we just fail the parse and don't try to
recover. I think losing error recovery in this obscure case is worth
the code cleanup.
This change affects `tests/ui/parser/impl-parsing.rs`, which is split in
two, and the obsolete `..` syntax cases are removed (they are tested
elsewhere).
