Allow parenthesis around inferred array lengths
In #135272 it was noticed that we weren't handling `Vec<(((((_)))))>` correctly under the new desugaring for `generic_arg_infer`, this had to be fixed in order to not regress stable code for types that should continue working. This has the side effect of *also* allowing the following to work:
```rust
#![feature(generic_arg_infer)]
struct Bar<const N: usize>;
fn main() {
let a: Bar<((_))> = Bar::<10>;
}
```
However I did not make the same change for array lengths resulting in the following not compiling:
```rust
#![feature(generic_arg_infer)]
fn main() {
let a: [u8; (((_)))] = [2; 2];
let a: [u8; 2] = [2; (((((_)))))];
}
```
This is rather inconsistent as parenthesis around `_` *are* supported for const args to non-arrays, and type args. This PR fixes this allowing the above example to compile. No stable impact.
r? compiler-errors
25d282efd4