…,oli-obk

Use undef for uninitialized bytes in constants

Fixes rust-lang#83657

This generates good code when the const is fully uninit, e.g.

```rust
#[no_mangle]
pub const fn fully_uninit() -> MaybeUninit<[u8; 10]> {
    const M: MaybeUninit<[u8; 10]> = MaybeUninit::uninit();
    M
}
```
generates
```asm
fully_uninit:
	ret
```

as you would expect.

There is no improvement, however, when it's partially uninit, e.g.

```rust
pub struct PartiallyUninit {
    x: u64,
    y: MaybeUninit<[u8; 10]>
}

#[no_mangle]
pub const fn partially_uninit() -> PartiallyUninit {
    const X: PartiallyUninit = PartiallyUninit { x: 0xdeadbeefcafe, y: MaybeUninit::uninit() };
    X
}
```
generates
```asm
partially_uninit:
	mov	rax, rdi
	mov	rcx, qword ptr [rip + .L__unnamed_1+16]
	mov	qword ptr [rdi + 16], rcx
	movups	xmm0, xmmword ptr [rip + .L__unnamed_1]
	movups	xmmword ptr [rdi], xmm0
	ret

.L__unnamed_1:
	.asciz	"\376\312\357\276\255\336\000"
	.zero	16
	.size	.L__unnamed_1, 24
```
which copies a bunch of zeros in place of the undef bytes, the same as before this change.

Edit: generating partially-undef constants isn't viable at the moment anyways due to rust-lang#84565, so it's disabled