Auto merge of #46808 - eddyb:issue-46769-quick, r=arielb1

rustc: ensure optimized enums have a properly aligned size.

Fixes #46769 by padding the optimized enums wrapping packed data as necessary.

Note that this is not the only way to solve this - on nightly, #46436 makes it easier to fix without adding new padding because of the replacement of `packed` flags with a non-redundant scheme.
But because it can't be backported, the optimal fix will be in a separate nightly-only PR (#46809).