…hallow resolvability

Only make GAT ambiguous in `match_projection_projections` considering shallow resolvability

In rust-lang#123537, I tweaked the hack from rust-lang#93892 to use `resolve_vars_if_possible` instead of `shallow_resolve`. This considers more inference guidance ambiguous. This resulted in crater regressions in rust-lang#125196.

I've effectively reverted the change to the old behavior. That being said, I don't *like* this behavior, but I'd rather keep it for now since rust-lang#123537 was not meant to make any behavioral changes. See the attached example.

This also affects the new solver, for the record, which doesn't have any rules about not guiding inference from param-env candidates which may constrain GAT args as a side-effect.

r? `@lcnr` or `@jackh726`

Add tests for `-Zunpretty=expanded` ported from stringify's tests

This PR adds a new set of tests for the AST pretty-printer.

Previously, pretty-printer edge cases were tested by way of `stringify!` in [tests/ui/macros/stringify.rs](https://github.com/rust-lang/rust/blob/1.78.0/tests/ui/macros/stringify.rs), such as the tests added by rust-lang@419b269 and rust-lang@527e2ea.

Those tests will no longer provide effective coverage of the AST pretty-printer after rust-lang#124141. `Nonterminal` and `TokenKind::Interpolated` are being removed, and a consequence is that `stringify!` will perform token stream pretty printing, instead of AST pretty printing, in all of the `stringify!` cases including $:expr and all other interpolations.

This PR adds 2 new ui tests with `compile-flags: -Zunpretty=expanded`:

- **tests/ui/unpretty/expanded-exhaustive.rs** — this test aims for exhaustive coverage of all the variants of `ExprKind`, `ItemKind`, `PatKind`, `StmtKind`, `TyKind`, and `VisibilityKind`. Some parts could use being fleshed out further, but the current state is roughly on par with what exists in the old stringify-based tests.

- **tests/ui/unpretty/expanded-interpolation.rs** — this test covers tricky macro metavariable edge cases that require the AST pretty printer to synthesize parentheses in order for the printed code to be valid Rust syntax.

r? `@nnethercote`

Clarify how String::leak and into_boxed_str differ