Add builtin functions for destroy, with special requirements in facet types (#6035)

This is in support of a goal of changing the blanket `destroy` impl to
use (roughly):

```
private fn CanAggregateDestroy() -> type = "type.can_aggregate_destroy";

// Handles aggregate type destruction.
impl forall [AggregateDestroyT:! CanAggregateDestroy()] AggregateDestroyT as Destroy {
  fn Op[addr self: Self*]() = "type.aggregate_destroy";
}
```

That isn't done here because there's still other issues that migrating
raises. What this *does* do is add the builtin functions, and in
particular, support to `FacetTypeInfo` to make `CanAggregateDestroy`
work.

The "special requirement" approach in `FacetTypeInfo` allows us to
support restricting a blanket impl under the current approach of impls.
Maybe we'll find a cleaner approach that can work in the future, but
this fits into the current model by propagating similar to other
requirements. I'm using an enum mask because we have a number of similar
things to add (e.g. copy, move) but I'm not sure we need a full vector.

A few alternatives considered were:

- Supporting syntax more like `where .Self impls
TypeCanAggregateDestroy(.Self, SupportedInterface,
UnsupportedInterface)`. I think it'd be a little cleaner, but requires
better compile-time evaluation in order to assess the type of the call.
Right now it's expected to be a `FacetType` too early to make this work,
and I was concerned about pouring too much more time down this route.
- Providing an actual interface, in particular doing name lookup back
into `Core.` for an interface. This would've added name lookup overhead,
and the question of whether an `impl` exists.
- Generating an interface. This avoids the name lookup, but would still
raise the question of whether an `impl` should also be generated. Work
I've previously done generating interfaces for class destruction also
feels complex to both write and understand (an unfortunate issue).
- Still modeling as an `ImplsConstraint`, for example by defining a
special `InterfaceId::CanAggregateDestroy = -2` similar to what we do on
other ids. I was hesitant because of how this expands the number of
modes of `InterfaceId`, and things for consuming code to watch out for,
for what feels like a relatively niche set of use-cases that are only
interface-like.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/check/eval.cpp

@@ -617,7 +617,10 @@ static auto GetConstantFacetTypeInfo(EvalContext& eval_context,
                                      SemIR::LocId loc_id,
                                      const SemIR::FacetTypeInfo& orig,
                                      Phase* phase) -> SemIR::FacetTypeInfo {
-  SemIR::FacetTypeInfo info;
+  SemIR::FacetTypeInfo info = {
+      .builtin_constraint_mask = orig.builtin_constraint_mask,
+      // TODO: Process other requirements.
+      .other_requirements = orig.other_requirements};
 
   info.extend_constraints.reserve(orig.extend_constraints.size());
   for (const auto& interface : orig.extend_constraints) {
@@ -661,9 +664,6 @@ static auto GetConstantFacetTypeInfo(EvalContext& eval_context,
     rewrite = {.lhs_id = lhs_id, .rhs_id = rhs_id};
   }
 
-  // TODO: Process other requirements.
-  info.other_requirements = orig.other_requirements;
-
   info.Canonicalize();
   return info;
 }
@@ -1646,7 +1646,8 @@ static auto MakeConstantForBuiltinCall(EvalContext& eval_context,
     case SemIR::BuiltinFunctionKind::None:
       CARBON_FATAL("Not a builtin function.");
 
-    case SemIR::BuiltinFunctionKind::NoOp: {
+    case SemIR::BuiltinFunctionKind::NoOp:
+    case SemIR::BuiltinFunctionKind::TypeAggregateDestroy: {
       // Return an empty tuple value.
       auto type_id = GetTupleType(eval_context.context(), {});
       return MakeConstantResult(
@@ -1656,6 +1657,18 @@ static auto MakeConstantForBuiltinCall(EvalContext& eval_context,
           phase);
     }
 
+    case SemIR::BuiltinFunctionKind::TypeCanAggregateDestroy: {
+      CARBON_CHECK(arg_ids.empty());
+      auto id = eval_context.facet_types().Add(
+          {.builtin_constraint_mask =
+               SemIR::BuiltinConstraintMask::TypeCanAggregateDestroy});
+      return MakeConstantResult(
+          eval_context.context(),
+          SemIR::FacetType{.type_id = SemIR::TypeType::TypeId,
+                           .facet_type_id = id},
+          phase);
+    }
+
     case SemIR::BuiltinFunctionKind::PrimitiveCopy: {
       return context.constant_values().Get(arg_ids[0]);
     }
@@ -2183,7 +2196,7 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
   auto typed_inst = inst.As<SemIR::WhereExpr>();
 
   Phase phase = Phase::Concrete;
-  SemIR::FacetTypeInfo info = {.other_requirements = false};
+  SemIR::FacetTypeInfo info;
 
   // Add the constraints from the `WhereExpr` instruction into `info`.
   if (typed_inst.requirements_id.has_value()) {
@@ -2204,6 +2217,7 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
           info.extend_constraints.append(base_info.extend_constraints);
           info.self_impls_constraints.append(base_info.self_impls_constraints);
           info.rewrite_constraints.append(base_info.rewrite_constraints);
+          info.builtin_constraint_mask.Add(base_info.builtin_constraint_mask);
           info.other_requirements |= base_info.other_requirements;
         }
       } else if (auto rewrite =
@@ -2242,6 +2256,7 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
             // Other requirements are copied in.
             llvm::append_range(info.rewrite_constraints,
                                more_info.rewrite_constraints);
+            info.builtin_constraint_mask.Add(more_info.builtin_constraint_mask);
             info.other_requirements |= more_info.other_requirements;
           }
         } else {
@@ -2249,7 +2264,7 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
           info.other_requirements = true;
         }
       } else {
-        // TODO: Handle other requirements
+        // TODO: Handle other requirements.
         info.other_requirements = true;
       }
     }