Add dedicated error message referring to cxx::ExternType when opaque type is used by value

[dtolnay]

The previous error of "error: passing non-trivial C++ type by value is not supported" was not sufficient to put someone on the right track of how to make their C++ type "trivial". The new error points to your type and very briefly explains what is needed and why, referring to cxx::ExternType where the documentation covers this use case in detail.

error: using opaque C++ type by value is not supported
 --> $DIR/by_value_not_supported.rs:3:16
  |
3 |     struct S { c: C }
  |                ^^^^

error: needs a cxx::ExternType impl in order to be used as a field of `S`
  --> $DIR/by_value_not_supported.rs:10:9
   |
10 |         type C;
   |         ^^^^^^

Follow-up to #325.