Remove InstId::Builtin members (#4632)

- `InstId::Builtin<Inst>` -> `<Inst>::SingletonInstId`
- `InstId::PackageNamespace` -> `Namespace::PackageInstId`

toolchain/check/member_access.cpp

@@ -143,17 +143,17 @@ static auto AccessMemberOfInterfaceWitness(
     -> SemIR::InstId {
   auto member_value_id = context.constant_values().GetConstantInstId(member_id);
   if (!member_value_id.is_valid()) {
-    if (member_value_id != SemIR::InstId::BuiltinErrorInst) {
+    if (member_value_id != SemIR::ErrorInst::SingletonInstId) {
       context.TODO(member_id, "non-constant associated entity");
     }
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
 
   auto assoc_entity =
       context.insts().TryGetAs<SemIR::AssociatedEntity>(member_value_id);
   if (!assoc_entity) {
     context.TODO(member_id, "unexpected value for associated entity");
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
 
   // TODO: This produces the type of the associated entity with no value for
@@ -181,7 +181,7 @@ static auto PerformImplLookup(
     context.TODO(loc_id,
                  "Lookup of impl witness not yet supported except for a single "
                  "interface");
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
 
   auto witness_id =
@@ -209,7 +209,7 @@ static auto PerformImplLookup(
                              interface_type_id,
                              context.GetTypeIdForTypeConstant(type_const_id));
     }
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
   return AccessMemberOfInterfaceWitness(context, loc_id, witness_id,
                                         interface_type->specific_id, assoc_type,
@@ -236,7 +236,7 @@ static auto LookupMemberNameInScope(Context& context, SemIR::LocId loc_id,
                                   /*required=*/true, access_info);
 
   if (!result.inst_id.is_valid()) {
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
 
   // TODO: This duplicates the work that HandleNameAsExpr does. Factor this out.
@@ -310,7 +310,7 @@ static auto LookupMemberNameInScope(Context& context, SemIR::LocId loc_id,
           context.TODO(member_id,
                        "associated entity not found in facet type, need to do "
                        "impl lookup");
-          return SemIR::InstId::BuiltinErrorInst;
+          return SemIR::ErrorInst::SingletonInstId;
         }
 
         member_id = AccessMemberOfInterfaceWitness(
@@ -432,7 +432,7 @@ auto PerformMemberAccess(Context& context, SemIR::LocId loc_id,
         return context.emitter().Build(base_id, IncompleteTypeInMemberAccess,
                                        base_id);
       })) {
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
 
   // Materialize a temporary for the base expression if necessary.
@@ -464,7 +464,7 @@ auto PerformMemberAccess(Context& context, SemIR::LocId loc_id,
                         SemIR::NameId);
       context.emitter().Emit(loc_id, QualifiedExprNameNotFound, base_id,
                              name_id);
-      return SemIR::InstId::BuiltinErrorInst;
+      return SemIR::ErrorInst::SingletonInstId;
     }
 
     if (base_type_id != SemIR::ErrorInst::SingletonTypeId) {
@@ -473,7 +473,7 @@ auto PerformMemberAccess(Context& context, SemIR::LocId loc_id,
                         TypeOfInstId);
       context.emitter().Emit(loc_id, QualifiedExprUnsupported, base_id);
     }
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
 
   // Perform lookup into the base type.
@@ -540,7 +540,7 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
                       "tuples can be indexed that way",
                       TypeOfInstId);
     context.emitter().Emit(loc_id, TupleIndexOnANonTupleType, tuple_inst_id);
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
 
   auto diag_non_constant_index = [&] {
@@ -548,7 +548,7 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
     CARBON_DIAGNOSTIC(TupleIndexNotConstant, Error,
                       "tuple index must be a constant");
     context.emitter().Emit(loc_id, TupleIndexNotConstant);
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   };
   // Diagnose a non-constant index prior to conversion to IntLiteral, because
   // the conversion will fail if the index is not constant.
@@ -563,7 +563,7 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
       context.GetBuiltinType(SemIR::BuiltinInstKind::IntLiteralType));
   auto index_const_id = context.constant_values().Get(index_inst_id);
   if (index_const_id == SemIR::ErrorInst::SingletonConstantId) {
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   } else if (!index_const_id.is_template()) {
     return diag_non_constant_index();
   }
@@ -574,7 +574,7 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
   std::optional<llvm::APInt> index_val = ValidateTupleIndex(
       context, loc_id, tuple_inst_id, index_literal, type_block.size());
   if (!index_val) {
-    return SemIR::InstId::BuiltinErrorInst;
+    return SemIR::ErrorInst::SingletonInstId;
   }
 
   // TODO: Handle the case when `index_val->getZExtValue()` has too many bits.