Rollup merge of #96162 - RalfJung:mark-uninit, r=oli-obk

interpret: Fix writing uninit to an allocation

When calling `mark_init`, we need to also be mindful of what happens with the relocations! Specifically, when we de-init memory, we need to clear relocations in that range as well or else strange things will happen (and printing will not show the de-init, since relocations take precedence there).

Fixes https://github.com/rust-lang/miri/issues/2068.

Here's the Miri testcase that this fixes (requires `-Zmiri-disable-validation`):
```rust
use std::mem::MaybeUninit;

fn main() { unsafe {
    let mut x = MaybeUninit::<i64>::uninit();
    // Put in a ptr.
    x.as_mut_ptr().cast::<&i32>().write_unaligned(&0);
    // Overwrite parts of that pointer with 'uninit' through a Scalar.
    let ptr = x.as_mut_ptr().cast::<i32>();
    *ptr = MaybeUninit::uninit().assume_init();
    // Reading this back should hence work fine.
    let _c = *ptr;
} }
```
Previously this failed with
```
error: unsupported operation: unable to turn pointer into raw bytes
  --> ../miri/uninit.rs:11:14
   |
11 |     let _c = *ptr;
   |              ^^^^ unable to turn pointer into raw bytes
   |
   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support

   = note: inside `main` at ../miri/uninit.rs:11:14
```

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