Auto merge of #1328 - RalfJung:align-false-pos, r=RalfJung

for alignment errors, note that there might be false positives

Cc @shepmaster

```
error: Undefined Behavior: accessing memory with alignment 1, but alignment 8 is required
 --> tests/compile-fail/unaligned_pointers/alignment.rs:8:9
  |
8 |         *y_ptr = 42;
  |         ^^^^^^^^^^^ accessing memory with alignment 1, but alignment 8 is required
  |
  = help: this usually indicates that your program performed an invalid operation and caused Undefined Behavior
  = help: but alignment errors can also be false positives, see #1074
```

src/diagnostics.rs

@@ -93,6 +93,11 @@ pub fn report_error<'tcx, 'mir>(
                     vec![format!("make sure to use a Miri sysroot, which you can prepare with `cargo miri setup`")],
                 Unsupported(_) =>
                     vec![format!("this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support")],
+                UndefinedBehavior(UndefinedBehaviorInfo::AlignmentCheckFailed { .. }) =>
+                    vec![
+                        format!("this usually indicates that your program performed an invalid operation and caused Undefined Behavior"),
+                        format!("but alignment errors can also be false positives, see https://github.com/rust-lang/miri/issues/1074"),
+                    ],
                 UndefinedBehavior(_) =>
                     vec![
                         format!("this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior"),