This change partially implements [PR #7362], which revises how objects
are destroyed. It is a partial implementation for two reasons:
1. This change moves `Destroy.Op`'s current behaviour into
`Destroy.SubobjectDestroy`, but it doesn't add support for objects with
non-trivial destruction.
2. `Destroy.SubobjectDestroy` is a workaround for `require impls
SubobjectDestroy`. We aren't able to use the latter until the dependents
add their requirements' implementations to their own witness tables.
[PR #7362]: https://github.com/carbon-language/carbon-lang/pulls/7362
Christopher Di Bella
f7cd39428e