Swift: remove Binding<> class

That class was meant to allow aggregate initialization of generated
C++ entries having the label `id` as first argument.

As aggregate initialization turned out to be undesirable (names of
fields are not explicit, and `{}` must be inserted for empty
superclasses), this commit removes it and disallows aggregate
initialization altogether by defining empty constructors for generated
classes.

swift/codegen/templates/cpp_classes.mustache

@@ -13,7 +13,12 @@
 namespace {{namespace}} {
 {{#classes}}
 
-struct {{name}}{{#final}} : Binding<{{name}}Tag>{{#bases}}, {{ref.name}}{{/bases}}{{/final}}{{^final}}{{#has_bases}}: {{#bases}}{{^first}}, {{/first}}{{ref.name}}{{/bases}}{{/has_bases}}{{/final}} {
+struct {{name}}{{#has_bases}} : {{#bases}}{{^first}}, {{/first}}{{ref.name}}{{/bases}}{{/has_bases}} {
+  {{name}}() {}
+
+  {{#final}}
+  {{trap_affix}}Label<{{name}}Tag> id{};
+  {{/final}}
   {{#fields}}
   {{type}} {{field_name}}{};
   {{/fields}}

swift/extractor/trap/TrapLabel.h

@@ -58,11 +58,6 @@ inline auto trapQuoted(const std::string& s) {
   return std::quoted(s, '"', '"');
 }
 
-template <typename Tag>
-struct Binding {
-  TrapLabel<Tag> id;
-};
-
 }  // namespace codeql
 
 namespace std {