Fix unused variable warnings across code generators

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.
See [upstream LLVM commit](https://github.com/llvm/llvm-project/commit/ab8b037ef7b398aa864959a69f328fa69b567f74) for reference.

- Remove unreferenced variable declarations whose initializers have no
  side effects (`generated_functions`, `offset_prefix`, `offset_size`,
  `filebase`, `field_name`, `offset`, `ts_file_path_rel`,
  `ts_file_path`, and `union_accessor`).
- Annotate variables initialized via helper or member calls with
  `[[maybe_unused]]` (`offset_str`, `get_field`, `qualified`,
  `struct_var`, `struct_type`, `type`, `def_mod_name`, and
  `ts_file_path`).

PiperOrigin-RevId: 999136370

src/idl_gen_cpp.cpp

@@ -2737,7 +2737,6 @@ class CppGenerator : public BaseGenerator {
     if (!field.IsScalarOptional()) {
       const bool is_scalar = IsScalar(type.base_type);
       std::string accessor;
-      std::string offset_size = "";
       if (is_scalar) {
         accessor = "GetField<";
       } else if (IsStruct(type)) {
@@ -2815,7 +2814,7 @@ class CppGenerator : public BaseGenerator {
 
   void GenTableFieldType(const FieldDef& field) {
     const auto& type = field.value.type;
-    const auto offset_str = GenFieldOffsetName(field);
+    [[maybe_unused]] const auto offset_str = GenFieldOffsetName(field);
     if (!field.IsScalarOptional()) {
       std::string afterptr = " *" + NullableExtension();
       code_.SetValue("FIELD_TYPE",