Assign a constant to ClassDecl/InterfaceDecl for name references. (#3722)

By adding a constant to ClassDecl/InterfaceDecl, we're able to remove
name reference special-casing. Use TryEvalInst on the Decl to generate
the Type. For ClassDecl, then use the generated constant for
self_type_id.

toolchain/check/eval.cpp

@@ -364,12 +364,30 @@ auto TryEvalInst(Context& context, SemIR::InstId inst_id, SemIR::Inst inst)
     case SemIR::TupleInit::Kind:
       return RebuildInitAsValue(context, inst, SemIR::TupleValue::Kind);
 
-    // These cases are always template constants.
     case SemIR::Builtin::Kind:
+      // Builtins are always template constants.
+      return MakeConstantResult(context, inst, Phase::Template);
+
+    case SemIR::ClassDecl::Kind:
+      // TODO: Once classes have generic arguments, handle them.
+      return MakeConstantResult(
+          context,
+          SemIR::ClassType{SemIR::TypeId::TypeType,
+                           inst.As<SemIR::ClassDecl>().class_id},
+          Phase::Template);
+
+    case SemIR::InterfaceDecl::Kind:
+      // TODO: Once interfaces have generic arguments, handle them.
+      return MakeConstantResult(
+          context,
+          SemIR::InterfaceType{SemIR::TypeId::TypeType,
+                               inst.As<SemIR::InterfaceDecl>().interface_id},
+          Phase::Template);
+
     case SemIR::ClassType::Kind:
     case SemIR::InterfaceType::Kind:
-      // TODO: Once classes and interfaces have generic arguments, handle them.
-      return MakeConstantResult(context, inst, Phase::Template);
+      CARBON_FATAL() << inst.kind()
+                     << " is only created during corresponding Decl handling.";
 
     // These cases are treated as being the unique canonical definition of the
     // corresponding constant value.
@@ -475,10 +493,8 @@ auto TryEvalInst(Context& context, SemIR::InstId inst_id, SemIR::Inst inst)
     case SemIR::Branch::Kind:
     case SemIR::BranchIf::Kind:
     case SemIR::BranchWithArg::Kind:
-    case SemIR::ClassDecl::Kind:
     case SemIR::ImplDecl::Kind:
     case SemIR::Import::Kind:
-    case SemIR::InterfaceDecl::Kind:
     case SemIR::Param::Kind:
     case SemIR::ReturnExpr::Kind:
     case SemIR::Return::Kind: