Add support to validity checks in the presence of copy_nonoverlapping, copy.

[celinval]

These intrinsics can affect the value of a place that may be read later using safe code. It is not immediate UB to write the value to the allocation space, but it would be to later use it with a type T where the value written is not valid.

E.g.:

use std::ptr;

/// # Safety
/// - Users must not read from `ptr` until a new valid value is written to it
unsafe fn copy_invalid_char(ptr: &mut char) {
    let invalid_value: u32 = 0xDE01;
    // SAFETY: This is
    unsafe { ptr::copy(&invalid_value, ptr as *mut _ as *mut u32, 1) };
}

/// This checks that there is no UB if the user ensures that a valid character is written before next read.
#[cfg_attr(kani, kani::proof)]
fn check_no_ub() {
    let mut c = 'a';
    // SAFETY: We will write to `c` before reading it again
    unsafe { copy_invalid_char(&mut c) };
    c = 'b';
    assert_ne!(c, 'a');
}

/// This checks that there is UB if a read exposes the invalid value.
#[cfg_attr(kani, kani::proof)]
fn check_with_ub() {
    let mut c = 'a';
    // SAFETY: This is not safe and it will trigger UB
    unsafe { copy_invalid_char(&mut c) };
    assert_ne!(c, 'a');
}

// Run this with MIRI
#[cfg(miri)]
fn main() {
    check_no_ub();
    check_with_ub();
}