Diagnose using named constraint's name inside its definition (#6906)

Using a named constraint inside itself is problematic:
- If there were not require decls written above, it identifies as an
empty set. This makes `Z(Self)` essentially disappear in the identified
facet type, which produces "no use of Self" diagnostics while the user
can see a use of Self in the code.
- It won't include require decls that are written after, and so `require
T impls Z` won't actually enforce that `T` impls all of `Z`.

Previously this was an error because using the named constraint would
require it to be identified, and it's not identified until it is
complete. But this will change in proposal #6902. So that proposal also
includes changes to preserve diagnostics for incorrect use of a named
constraint before it's complete, which is implemented here.

Discussed in open discussion [on
2026-03-12](https://docs.google.com/document/d/1mjllGO3ZCL4qGt9uJHUtcxKoHAGEY7Y999ie4EtBWB8/edit?tab=t.0#heading=h.1dvbbrp5a6t3).

The new tests exposed a bug where we're not copying named constraints in
a facet type on the RHS of `where .Self impls` into the facet type on
the left, which is now fixed. The
`fail_require_impls_incomplete_self_in_period_self_impls.carbon` test
would not diagnose its error without this fix.

toolchain/check/handle_require.cpp

@@ -17,6 +17,7 @@
 #include "toolchain/parse/node_ids.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/named_constraint.h"
+#include "toolchain/sem_ir/specific_named_constraint.h"
 #include "toolchain/sem_ir/type_iterator.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
@@ -209,6 +210,36 @@ static auto ValidateRequire(Context& context, SemIR::LocId loc_id,
     return std::nullopt;
   }
 
+  if (auto named_constraint =
+          context.insts().TryGetAs<SemIR::NamedConstraintWithSelfDecl>(
+              scope_inst_id)) {
+    const auto& constraint_facet_type_info =
+        context.facet_types().Get(constraint_facet_type->facet_type_id);
+    // TODO: Handle other impls named constraints for the
+    // RequireImplsReferenceCycle diagnostic.
+    if (constraint_facet_type_info.other_requirements) {
+      context.TODO(constraint_inst_id,
+                   "facet type has constraints that we don't handle yet");
+      return std::nullopt;
+    }
+    auto named_constraints = llvm::concat<const SemIR::SpecificNamedConstraint>(
+        constraint_facet_type_info.extend_named_constraints,
+        constraint_facet_type_info.self_impls_named_constraints);
+    for (auto c : named_constraints) {
+      if (c.named_constraint_id == named_constraint->named_constraint_id) {
+        const auto& named_constraint =
+            context.named_constraints().Get(c.named_constraint_id);
+        CARBON_DIAGNOSTIC(RequireImplsReferenceCycle, Error,
+                          "facet type in `require` declaration refers to the "
+                          "named constraint `{0}` from within its definition",
+                          SemIR::NameId);
+        context.emitter().Emit(constraint_inst_id, RequireImplsReferenceCycle,
+                               named_constraint.name_id);
+        return std::nullopt;
+      }
+    }
+  }
+
   auto identified_facet_type_id = RequireIdentifiedFacetType(
       context, SemIR::LocId(constraint_inst_id), self_constant_value_id,
       *constraint_facet_type, [&](auto& builder) {