Rollup merge of #64690 - petrochenkov:mixed, r=dtolnay

proc_macro API: Expose `macro_rules` hygiene

Proc macros do not have direct access to our oldest and most stable hygiene kind - `macro_rules` hygiene.

To emulate it macro authors have to go through two steps - first generate a temporary `macro_rules` item (using a derive, at least until https://github.com/rust-lang/rust/pull/64035 is merged), then generate a macro call to that item. Popular crates like [proc_macro_hack](https://crates.io/crates/proc-macro-hack) use this trick to generate hygienic identifiers from proc macros.

I'd say that these workarounds with nested macro definitions have more chances to hit some corner cases in our hygiene system, in which we don't have full confidence.
So, let's provide a direct access to `macro_rules` hygiene instead.

This PR does that by adding a new method `Span::mixed_site` (bikeshedding is welcome) in addition to existing `Span::call_site` (stable) and `Span::def_site` (unstable).
Identifiers with this span resolve at def-site in for local variables, labels and `$crate`, and resolve at call-site for everything else, i.e. exactly like identifiers produced by `macro_rules`.

This API addition opens the way to stabilizing proc macros in expression positions (https://github.com/rust-lang/rust/issues/54727), for which use of call-site hygiene or workarounds with temporary items would be quite unfortunate.
(`macro_rules` expanded in expression position, on the other hand, are stable since 1.0 and widely used.)

r? @dtolnay @alexcrichton