generation of non-compilable C++

[wds15]

The attached Stan program is a legitimate Stan2 program which just compiles to C++ and then to a binary; running just fine with 2.20. With stanc3 a C++ code is generated which cannot be compiled.

This is a slightly modified code from the overpropagation to var issue, but this time focusing on a different aspect.

oral_1cmt_mm_run_stan.txt

In case you want to run the model, just take the data and seed from the issue #336