`let_unit_with_type_underscore`: make early-pass (#15458)

This kind of supersedes
https://github.com/rust-lang/rust-clippy/pull/15386 -- by making the
lint early-pass, we get access to `TyKind::Paren`s that surround the
type ascription. And with that, we can return to the simpler calculation
of `span_to_remove`.

The biggest hurdle was `is_from_proc_macro` -- calling that function
required me to `impl WithSearchPat for rustc_ast::Ty`, i.e.
`ast_ty_search_pat`, which I based on `ty_search_pat`. Since that's a
larger change, I could extract it into a PR of its own.

changelog: none

0 files changed, 0 insertions, 0 deletions