Emit explanatory note for move errors in packed struct derives
Derive expansions for packed structs with non-`Copy` fields cause move errors because they prefer copying over borrowing since borrowing the fields of a packed struct can result in unaligned access.
This underlying cause of the errors, however, is not apparent to the user. This PR adds a diagnostic note to make it clear to the user (the new note is on the second last line):
```
tests/ui/derives/deriving-with-repr-packed-move-errors.rs:13:16
|
12 | #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Default)]
| ----- in this derive macro expansion
13 | struct StructA(String);
| ^^^^^^ move occurs because `self.0` has type `String`, which does not implement the `Copy` trait
|
= note: `#[derive(Debug)]` triggers a move because taking references to the fields of a packed struct is undefined behaviour
= note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
```
Fixes#117406
Partially addresses #110777
7b97a5ca84