Add moveable_types=forward_setter option for perfect forwarding setters

[zsy056]

Description

Adds forward_setter value to moveable_types option, generating perfect forwarding setters for complex types while preserving traditional setters for primitives. Also fixes missing operator< implementation that caused link errors when structs are used as map keys.

Generator Changes

Forward Setter Generation (compiler/cpp/src/thrift/generate/t_cpp_generator.cc):

• Parse moveable_types=forward_setter option
• Complex types (strings, containers, structs) → template setters with std::forward<T_>
• Primitive types → traditional const-ref setters
• Template implementations in .tcc file (auto-included in header)
• Legacy moveable_types behavior unchanged

Testing

Compiler Unit Tests (compiler/cpp/tests/cpp/):

• New test_forward_setter.thrift fixture
• Dedicated t_cpp_generator_forward_setter_tests.cc (91 assertions, 9 test cases)
• Verify .tcc generation and template implementations

Integration Tests (test/cpp/src/):

• ForwardSetterTest.cpp - validates lvalue/rvalue/temporary/literal setters with move semantics
• PrivateOptionalTest.cpp - SFINAE + static_assert verify optional fields are private
• EnumClassTest.cpp - type_traits + static_assert verify true enum class semantics

Build System

CMakeLists.txt (test/cpp/):

• Separate gen-cpp-{forward,private,enumclass} directories

Makefile.am (test/cpp/):

• Library targets for each option variant
• Proper BUILT_SOURCES dependencies
• Include path ordering: option-specific directory before standard gen-cpp

Example

// Generated with --gen cpp:moveable_types=forward_setter

struct TestStruct {
  int32_t primitive_field;
  std::string complex_field;
  
  void __set_primitive_field(const int32_t val);  // Traditional
  
  template <typename T_>
  void __set_complex_field(T_&& val);  // Perfect forwarding
};

// In .tcc file:
template <typename T_>
void TestStruct::__set_complex_field(T_&& val) {
  this->complex_field = ::std::forward<T_>(val);
  __isset.complex_field = true;
}

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[Jens-G]

Couild you please remove the [skip ci] piece from your commit message?

[zsy056]

Couild you please remove the [skip ci] piece from your commit message?

thanks, pushed an update.