Make cxx.h available to high level code generators.

[adetaylor]

At present, this file is available only from the git repository,
but higher level code generators may wish to ensure that they supply
a version of cxx.h corresponding precisely to the version of cxx
in use.

Specifically, such higher level code generators may wish to use
rust::Str and similar types in code which they autogenerate,
and thus need a way to include definitions of such types. As
this code is autogenerated, it can't reasonably rummage around
the cxx git repository to find the correct cxx.h header.

To be even more specific, higher level code generators may wish
to pass rust::Str and/or rust::String types into C++, in order
to create UniquePtr<CxxString>s from Rust strings during function
calls.

[dtolnay]

Thanks!

I think we might as well expose this as a static rather than a method call. I don't have plans to move away from the include_str!-based implementation.