Add Destroy.SubobjectDestroy as a temporary replacement for Destroy.Op (#7773)

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

toolchain/check/cpp/operators.cpp

@@ -39,6 +39,8 @@ static auto GetClangOperatorKind(Context& context, SemIR::LocId loc_id,
   switch (interface_name) {
       // Unary operators.
     case CoreIdentifier::Destroy:
+    case CoreIdentifier::SubobjectDestroy:
+    case CoreIdentifier::SelfDestruct:
     case CoreIdentifier::As:
     case CoreIdentifier::ImplicitAs:
     case CoreIdentifier::Iterate: