Removing _LIBCPP_DISABLE_UNUSED_STRUCT_WARNINGS from libc++ enables

`[[gnu::__warn_unused__]]` on standard library types (such as
`std::string` and `std::set`), surfacing `-Wunused-variable` warnings
in `//third_party/flatbuffers`. See [upstream LLVM commit](https://github.com/llvm/llvm-project/commit/ab8b037ef7b398aa864959a69f328fa69b567f74) for reference.

Remove unreferenced variable declarations across the code generators and
binary annotator whose initializers have no observable side effects.

PiperOrigin-RevId: 999136370

src/idl_gen_java.cpp

@@ -2176,6 +2176,7 @@ class JavaGenerator : public BaseGenerator {
       if (field.deprecated) continue;
       if (field.value.type.base_type == BASE_TYPE_UTYPE) continue;
       if (field.value.type.element == BASE_TYPE_UTYPE) continue;
+      const auto get_field = namer_.Method("get", field);
 
       code += "    this." + namer_.Field(field) + " = ";
       const auto type_name =

src/idl_gen_swift.cpp

@@ -1141,6 +1141,7 @@ class SwiftGenerator : public BaseGenerator {
          it != struct_def.fields.vec.end(); ++it) {
       const auto& field = **it;
       if (field.deprecated) continue;
+      const auto offset = NumToString(field.value.offset);
 
       code_.SetValue("FIELDVAR", namer_.Variable(field));
       code_.SetValue("VALUETYPE", GenerateVerifierType(field));