Initial support for empty named constraints (#6245)

Type check named constraint decls and definitions. We don't correctly
error if you put a `fn` inside them. There is no support for `require`
or `alias` yet, so there's nothing useful you can do with them yet.

We have attempted to share code between `interface` and `constraint` as
they are quite similar. First by splitting out some of
handle_interface.cpp to a separate file. Second by sharing some code
paths when you want a facet type from either one, as they both turn into
a facet type.

toolchain/sem_ir/stringify.cpp

@@ -437,6 +437,16 @@ class Stringifier {
                       ">");
   }
 
+  auto StringifyInst(InstId /*inst_id*/, GenericNamedConstraintType inst)
+      -> void {
+    const auto& constraint =
+        sem_ir_->named_constraints().Get(inst.named_constraint_id);
+    *out_ << "<type of ";
+    step_stack_->Push(StepStack::QualifiedNameItem{constraint.parent_scope_id,
+                                                   constraint.name_id},
+                      ">");
+  }
+
   // Determine the specific interface that an impl witness instruction provides
   // an implementation of.
   // TODO: Should we track this in the type?