Add an interface-with-self generic to each interface and same for constraints (#6667)

Currently each interface has a `Self` facet internally that becomes a
binding to every entity inside the interface: associated constants,
functions, and require decls. Each of these has to be independently
generic as a result. This makes is challenging in extended name lookup
to move into an extended scope of an interface, as we have a specific
for the interface, but the names within require a different specific
that includes a `Self` facet value.

We generalize this relationship by adding a second generic to Interface,
called `generic_with_self`. When we want to work with entities inside
the interface, we move from the interface-without-specific to the
interface-with-self specific by adding a Self to the specific. This is
done independently of any particular entity inside the Interface, as
those entities are now all members of the interface-with-self generic.

Associated constants no longer need a generic of their own, as they do
not have separate generic bindings. Functions retain a generic, but if
the function has no generic arguments, it will have no bindings of its
own now.

Require decls retain a generic so that their specific can be
instantiated separately from the interface. Requiring the interface to
be complete does not require the types in a require decl to be complete
unless it is modified by `extend`. So we allow them to be completed
later by keeping them in a separate generic.

Named constraints look like interfaces and gain the additional inner
generic-with-self, with the same relationship to require decls.

This removes the need for name lookup to perform Substitution of a Self
facet into the extended scope instruction. Instead, the
`SpecificConstant` instruction inserted by a `require` decl is part of
the interface-with-self generic. When looking through a FacetType for
extended scopes, for each interface, we push the scope with the specific
for the interface-with-self. Then the constant value of the
`SpecificConstant` is correctly modified by the provided self
automatically through applying that specific.

toolchain/sem_ir/stringify.cpp

@@ -214,8 +214,22 @@ class StepStack {
       return;
     }
     const auto& specific = sem_ir_->specifics().Get(specific_id);
-    auto args =
-        sem_ir_->inst_blocks().Get(specific.args_id).take_back(num_params);
+
+    auto drop_back_count = 0;
+    const auto& generic = sem_ir_->generics().Get(specific.generic_id);
+    if (sem_ir_->insts()
+            .IsOneOf<InterfaceWithSelfDecl, NamedConstraintWithSelfDecl>(
+                generic.decl_id)) {
+      // The with-self generic contains an additional `Self` parameter beyond
+      // the parameters of the entity, the argument for which we do not include
+      // in the display.
+      drop_back_count = 1;
+    }
+
+    auto args = sem_ir_->inst_blocks()
+                    .Get(specific.args_id)
+                    .drop_back(drop_back_count)
+                    .take_back(num_params);
     bool last = true;
     for (auto arg : llvm::reverse(args)) {
       PushString(last ? ")" : ", ");
@@ -288,8 +302,7 @@ class Stringifier {
   auto StringifyInst(InstId /*inst_id*/, AssociatedEntityType inst) -> void {
     *out_ << "<associated entity in ";
     step_stack_->Push(">");
-    step_stack_->PushSpecificInterface(
-        SpecificInterface{inst.interface_id, inst.interface_specific_id});
+    step_stack_->PushSpecificInterface(inst.GetSpecificInterface());
   }
 
   auto StringifyInst(InstId /*inst_id*/, BoolLiteral inst) -> void {
@@ -823,12 +836,25 @@ auto StringifySpecific(const File& sem_ir, SpecificId specific_id)
           sem_ir.interfaces().Get(interface_decl.interface_id), specific_id);
       break;
     }
+    case CARBON_KIND(InterfaceWithSelfDecl interface_with_self_decl): {
+      step_stack.PushEntityName(
+          sem_ir.interfaces().Get(interface_with_self_decl.interface_id),
+          specific_id);
+      break;
+    }
     case CARBON_KIND(NamedConstraintDecl constraint_decl): {
       step_stack.PushEntityName(
           sem_ir.named_constraints().Get(constraint_decl.named_constraint_id),
           specific_id);
       break;
     }
+    case CARBON_KIND(NamedConstraintWithSelfDecl constraint_with_self_decl): {
+      step_stack.PushEntityName(
+          sem_ir.named_constraints().Get(
+              constraint_with_self_decl.named_constraint_id),
+          specific_id);
+      break;
+    }
     case CARBON_KIND(RequireImplsDecl _): {
       step_stack.Push("require");
       break;