Remove most of the metaprogramming in node.h in favor of listing all the members in the typed node structs. (#3310)

Split `node.h` into separate files for ID types (`id.h`) and for typed
nodes (`typed_nodes.h`). The per-node-kind data is now specified as part
of declaring the typed nodes, and is removed from the node kinds
x-macros, which now simply enumerate the node kinds.

toolchain/check/handle_if_statement.cpp

@@ -53,7 +53,7 @@ auto HandleIfStatement(Context& context, Parse::Node parse_node) -> bool {
       // block.
       auto else_block_id =
           context.node_stack().Pop<Parse::NodeKind::IfCondition>();
-      context.AddNode(SemIR::Branch(parse_node, else_block_id));
+      context.AddNode(SemIR::Branch{parse_node, else_block_id});
       context.node_block_stack().Pop();
       context.node_block_stack().Push(else_block_id);
       break;