define_aggregate: provide empty user-defined constructor or destructor for unions with non-trivial members

[BaLiKfromUA]

Issue number of the reported bug or feature request: #115

Describe your changes

Tried to implement next behaviour from paper:

[7.10] If C is a union type for which any of its members are not trivially default constructible, then D has a user-provided default constructor which has no effect.

[7.11] If C is a union type for which any of its members are not trivially destructible, then D has a user-provided destructor which has no effect.  

Testing performed

• Automated testing with type_traits
• Manual testing via code like this

union UU;
static_assert(is_type(define_aggregate(
    ^^UU,
    {
        data_member_spec(^^int),
        data_member_spec(^^std::string),
    })));

int main() {
  UU u;
  u.~UU();
  
  return 0;
}

• assertations in code

Additional context

N/A

[BaLiKfromUA]

potentially hacky approach but I tried to reuse existing code as much as possible to not copy generation of constructors

[BaLiKfromUA]

decided to put this logic here instead of SemaReflect because this code is specific to function and doesn't require access to Sema