Friendlier message when cxx::CxxString is used in a #[cxx::bridge].

[anforowicz]

One gotcha that sometimes trips new users of cxx is that cxx::CxxString works outside of cxx::bridge, but not within.
This PR tries to improve this by providing a custom error message in this scenario.

Notes:

• I think that any custom error message is a minor improvement over the status quo (with a generic "unsupported type" error message). Additional improvements may be possible, but it's probably okay to do them incrementally later, in separate PRs.
  • Feel free to tweak the error message as needed. I wasn't sure if it's okay to call cxx:: a "qualifier" but it seems okay (FWIW paths containing :: are called "qualified paths" in Rust reference).
  • Ideally the error message would also provide a hint (i.e. a fix suggestion) that enables automating removal of cxx::. Unfortunately, I wasn't able to figure out how to do it. It seems that syn::Error doesn't support fix hints. And it seems that compiler folks say that annotate_snippets may be one option, but adopting this in cxx seems like a potentially bigger work item?
• FWIW an alternative fix here would be to accept both cxx::CxxString and CxxString. I think that having a single spelling is somewhat more desirable (for grepping, etc.), but I am open to implementing the alternative fix.

/cc @zetafunction, @adetaylor

[dtolnay]

Thanks!