Switch types to a SemanticsTypeId. (#2854)

This switches types to using SemanticsTypeId instead of SemanticsNodeId, and lowering pre-builds its list of types. The empty tuple type is special-cased because we don't want to emit it unless it's in-use, but as the implicit return for functions, it's frequently used. Callables use invalid to indicate the implicit return, and that seems undesirable to change due to the size increase.

toolchain/semantics/semantics_handle_struct.cpp

@@ -28,7 +28,7 @@ auto SemanticsHandleStructFieldDesignator(SemanticsContext& context,
 auto SemanticsHandleStructFieldType(SemanticsContext& context,
                                     ParseTree::Node parse_node) -> bool {
   auto [type_node, type_id] = context.node_stack().PopForParseNodeAndNodeId();
-  SemanticsNodeId cast_type_id = context.ExpressionAsType(type_node, type_id);
+  SemanticsTypeId cast_type_id = context.ExpressionAsType(type_node, type_id);
 
   auto [name_node, name_id] = context.node_stack().PopForParseNodeAndNameId(
       ParseNodeKind::DesignatedName);
@@ -80,9 +80,7 @@ auto SemanticsHandleStructLiteral(SemanticsContext& context,
   auto refs = context.semantics().GetNodeBlock(refs_id);
   auto type_id =
       context.CanonicalizeType(context.AddNode(SemanticsNode::StructType::Make(
-          parse_node,
-          context.CanonicalizeType(SemanticsNodeId::BuiltinTypeType),
-          type_block_id)));
+          parse_node, SemanticsTypeId::TypeType, type_block_id)));
 
   auto value_id = context.AddNode(
       SemanticsNode::StructValue::Make(parse_node, type_id, refs_id));
@@ -117,8 +115,7 @@ auto SemanticsHandleStructTypeLiteral(SemanticsContext& context,
       << "{} is handled by StructLiteral.";
 
   auto type_id = context.AddNode(SemanticsNode::StructType::Make(
-      parse_node, context.CanonicalizeType(SemanticsNodeId::BuiltinTypeType),
-      refs_id));
+      parse_node, SemanticsTypeId::TypeType, refs_id));
   context.node_stack().Push(parse_node, type_id);
   return true;
 }