This commit does not change how the incremental accounting is done, so
changes (or accessses) to an impl-item are still tagged to the enclosing
impl.

This commits adds the "main guts" of this change. It does not build on
its own.

There are now three patterns (shallow, deep, and nested visit).  These
are described in detail on the docs in `itemlikevisit::ItemLikeVisitor`.

This speeds up compilation by 3--6% across most of rustc-benchmarks.

On 64-bit platforms this reduces the size of `Expr_` from  64 bytes to
56 bytes, and reduces the size of `Expr` from 88 bytes to 80 bytes.

On 64-bit platforms this reduces the size of `Expr` from 96 bytes to 88
bytes.

On 64-bit this reduces the size of `Expr_` from 144 to 64 bytes, and
reduces the size of `Expr` from 176 to 96 bytes.

and refacotor `hir::PathListItem_::Ident` -> `hir::PathListItem_`.

This avoids the compile-time overhead of computing them twice.  It also fixes
an issue where the hash computed after typeck is differen than the hash before,
because typeck mutates the def-map in place.

Fixes #35549.
Fixes #35593.

Split Ty::is_empty method into is_never and is_uninhabited

Replace FnOutput with Ty
Replace FnConverging(ty) with ty
Purge FnDiverging, FunctionRetTy::NoReturn and FunctionRetTy::None

Add `TyKind::Empty` and fix resulting build errors.

Refactored tokentrees into their own files in preparation for tokenstreams. Modified tests to point to the new file now.

This makes the "shadowing labels" warning *not* print the entire loop
as a span, but only the lifetime.

Also makes #31719 go away, but does not fix its root cause (the span
of the expanded loop is still wonky, but not used anymore).