UnPack should not return a bare pointer [C++]

[DaveBrantonCTCT]

In the generated code for UnPack, a unique_ptr is used internally - which was done in #5761 to prevent a leak in an OOM condition.

However, the signature of the function still returns a bare pointer. Perhaps this was left as-is for compatibility reasons, but this code would be much better if it returned the unique_ptr instead.

If the compatibility break would be too onerous for C++ users, then this behaviour could be behind a commandline option. I personally doubt this in any case since modern C++ code should immediately wrap this return value in a smart pointer, which would still compile with the change.