[SwiftSyntax] Fix issue in generation of ParsedSyntaxBuilders

`child_elt_name` was inferred from the last iteration of the loop above, which was incorrect.

Author: ahoppen

lib/Parse/ParsedSyntaxBuilders.cpp.gyb

@@ -85,6 +85,7 @@ void Parsed${node.name}Builder::finishLayout(bool deferred) {
 % if node.children:
 %   for (idx, child) in enumerate(node.children):
 %     child_elt = None
+%     child_elt_name = child.name + 'Member'
 %     child_node = NODE_MAP.get(child.syntax_kind)
 %     if child_node and child_node.is_syntax_collection():
 %       child_elt = child_node.collection_element_name