Make DMD emit C++ .h files same as .di files

[Manu]

I've been talking about this for years, and logged this a while back:
https://issues.dlang.org/show_bug.cgi?id=19579

Is there anyone interested in or knows how to do this?
It would really be super valuable; this idea would motivate
inter-language projects that typically go C++-fist-with-a-D-binding to
work the other way.

Creating a pressure to write D-code first because the binding part is
maintained automatically when you compile is potentially significant;
gets programmers thinking and writing D code as first-class.

Idea would be same as emitting .di files, but in this case only
extern(C)/extern(C++) declarations would be emit to a .h file.

I could use this to create some nice demos in my office which I think
would get a few people talking and excited.