mgca: const array diagnostics aren't great

followup from https://github.com/rust-lang/rust/pull/162845#pullrequestreview-5221186941

there's at least three ways that array diagnostics are bad, in my opinion:

1. [u8] is unconditionally formatted as a *b""

#![feature(min_adt_const_params, min_generic_const_args)]
struct S<const A: [u8; 2]>;
fn main() {
    let a: S<core::direct_const_arg!([104, 105])> = S::<core::direct_const_arg!([110, 111])>;
}

error[E0308]: mismatched types
  --> /d/rust/tests/ui/asdf.rs:4:53
   |
LL |     let a: S<core::direct_const_arg!([104, 105])> = S::<core::direct_const_arg!([110, 111])>;
   |            --------------------------------------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `*b"hi"`, found `*b"no"`
   |            |
   |            expected due to this
   |
   = note: expected struct `S<*b"hi">`
              found struct `S<*b"no">`

this is due to

https://github.com/rust-lang/rust/blob/28e8a8c81bf3b37909edac6c2a76e56f30cd492f/compiler/rustc_middle/src/ty/print/pretty.rs#L1910-L1918

2. types are not printed, "expected [1], found [1]" is confusing in the below message. (unsure if the first message can be silenced or something, if the second message always appears. this is due to https://github.com/rust-lang/rust/pull/162845 and there's probably better ways of handling this, not producing additional errors about a not-wf ty, or something)

#![feature(min_adt_const_params, min_generic_const_args)]
struct S<const A: [u16; 1]>;
fn main() {
    let a: S<core::direct_const_arg!([1_u16])> = S::<core::direct_const_arg!([1_u32])>;
}

error[E0308]: mismatched types
  --> /d/rust/tests/ui/asdf.rs:4:50
   |
LL |     let a: S<core::direct_const_arg!([1_u16])> = S::<core::direct_const_arg!([1_u32])>;
   |            -----------------------------------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `[1]`, found `[1]`
   |            |
   |            expected due to this
   |
   = note: expected struct `S<[1]>`
              found struct `S<[1]>`

error: the constant `1` is not of type `u16`
  --> /d/rust/tests/ui/asdf.rs:4:78
   |
LL |     let a: S<core::direct_const_arg!([1_u16])> = S::<core::direct_const_arg!([1_u32])>;
   |                                                                              ^^^^^^^ expected `u16`, found `u32`

3. length mismatches print the values of every element in the whole array, they don't just print the length mismatch

#![feature(min_adt_const_params, min_generic_const_args)]
struct S<const A: [u32; 5]>;
fn main() {
    let a = S::<core::direct_const_arg!([1, 2, 3, 4])>;
}

error: the constant `[1, 2, 3, 4]` is not of type `[u32; 5]`
  --> /d/rust/tests/ui/asdf.rs:4:13
   |
LL |     let a = S::<core::direct_const_arg!([1, 2, 3, 4])>;
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `[u32; 5]`, found `[u32; 4]`
   |
note: required by a const generic parameter in `S`
  --> /d/rust/tests/ui/asdf.rs:2:10
   |
LL | struct S<const A: [u32; 5]>;
   |          ^^^^^^^^^^^^^^^^^ required by this const generic parameter in `S`

fyi @BoxyUwU

Start with the linked review and the referenced formatting logic in compiler/rustc_middle/src/ty/print/pretty.rs, then reproduce the three examples from the issue in UI tests. Compare diagnostics for byte-array formatting, element types, and length mismatches; done means the resulting errors distinguish the relevant types and report length mismatches without unnecessarily printing every element.