handle the distinction of Unit and Void in C/C++ code gen

[feiwang3311]

Guannan says:
For my own stuff I have patch for C++:
override def quote(s: Def): String = s match {
case Const(()) => "std::monostate{}";
case _ => super.quote(s)
}
and
override def primitive(t: String): String = t match {
case "Unit" => "std::monostate"
case _ => super.primitive(t)
}
I think we can either 1) pretend Scala Unit is C void, which is the current status 2) or add some macro of Unit in C (eg, define unit value is char 0), 3) or add a Rep[Void] type in Scala/LMS which correspond to C void type (2) requires changes of the generated code (as well as existing projects and tests)(3) requires changes from the scala meta programs