CPP12 — Structure member ordering

The C++ mapping says in section 1.10:

An OMG IDL struct maps to C++ struct, with each OMG IDL struct
member mapped to a corresponding member of the C++ struct.

This doesn't say that the order of the struct members in C++ must be the
same as in the IDL. Without that guarantee, code that compares pointers to
members is non-portable. We need a requirement to preserve the order here.

see below