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/check/modifiers.cpp

@@ -221,10 +221,12 @@ auto RestrictExternModifierOnDecl(Context& context,
   }
 }
 
-auto RequireDefaultFinalOnlyInInterfaces(
-    Context& context, DeclIntroducerState& introducer,
-    std::optional<SemIR::Inst> parent_scope_inst) -> void {
-  if (parent_scope_inst && parent_scope_inst->Is<SemIR::InterfaceDecl>()) {
+auto RequireDefaultFinalOnlyInInterfaces(Context& context,
+                                         DeclIntroducerState& introducer,
+                                         SemIR::NameScopeId parent_scope_id)
+    -> void {
+  if (parent_scope_id.has_value() &&
+      context.name_scopes().Get(parent_scope_id).is_interface_definition()) {
     // Both `default` and `final` allowed in an interface definition.
     return;
   }