Auto merge of #134424 - 1c3t3a:null-checks, r=saethlin

Insert null checks for pointer dereferences when debug assertions are enabled

Similar to how the alignment is already checked, this adds a check
for null pointer dereferences in debug mode. It is implemented similarly
to the alignment check as a `MirPass`.

This inserts checks in the same places as the `CheckAlignment` pass and additionally
also inserts checks for `Borrows`, so code like
```rust
let ptr: *const u32 = std::ptr::null();
let val: &u32 = unsafe { &*ptr };
```
will have a check inserted on dereference. This is done because null references
are UB. The alignment check doesn't cover these places, because in `&(*ptr).field`,
the exact requirement is that the final reference must be aligned. This is something to
consider further enhancements of the alignment check.

For now this is implemented as a separate `MirPass`, to make it easy to disable
this check if necessary.

This is related to a 2025H1 project goal for better UB checks in debug
mode: https://github.com/rust-lang/rust-project-goals/pull/177.

r? `@saethlin`

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -508,6 +508,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                     found: eval_to_int(found)?,
                 }
             }
+            NullPointerDereference => NullPointerDereference,
         };
         Err(ConstEvalErrKind::AssertFailure(err)).into()
     }