Parse const blocks in path arguments

- Part of https://github.com/rust-lang/rust/issues/132980

Example: https://github.com/rust-lang/rust/blob/d39561c2d2b55985d3b6331cc52403e038e7fc8b/tests/ui/const-generics/mgca/higher-ranked-lts-good.rs#L10

```rust
fn foo()
where
T: Trait fn(&'a ()); 1 }>
{}
```

Example: https://github.com/rust-lang/rust/blob/d39561c2d2b55985d3b6331cc52403e038e7fc8b/tests/ui/traits/associated_type_bound/generic-const-args-default.rs#L6

```rust
trait Bar {}
```

Start with the two linked Rust examples: tests/ui/const-generics/mgca/higher-ranked-lts-good.rs and tests/ui/traits/associated_type_bound/generic-const-args-default.rs, then trace syn's path-argument parsing entry point. Done means both const-block forms parse successfully without regressing existing path and generic-argument cases.

Written by the indexing model from the issue text.