C++ module compilation and linking support.

[JCBurnside]

I would like to use c++ modules for the c++ code side but atm it's not supported by cxx-bridge or cxx-build. I can work around it like

module;
#include "header.h"
export module foo;

export extern "C++" void foo::example() {
  ...
}

and that allows for the names to be mangled the same way as if it were non-module c++ code. I can imagine it'd be a simple flag for the cxx-bridge executable though could present an issue for opaque c++ types. But am not sure how to handle it for the cxx-build as afaict there is no c++ module support in the cc crate.