Parser accepts negative lifetime bounds and interprets as positive bound

The following is accepted by the parser:
```rust
fn foo<T: !'static>() {}
```
and the `!` is just silently dropped thus interpreting the constraint as `T: 'static`.

Regression introduced in https://github.com/rust-lang/rust/pull/57364 on Feb 24.
cc @estebank @hdhoang

I discovered this while working on a refactoring of `parser/ty.rs`.
I will fix the issue as part of that refactoring.