Noncompilable C++ code if method has different explicit namespace than the receiver

[dtolnay]

As observed in #441 (comment):

#[cxx::bridge]
mod ffi {
    #[namespace = "shared"]
    struct Color {
        r: u8,
        g: u8,
        b: u8,
    }

    #[namespace = "rust_part"]
    extern "Rust" {
        fn is_white(self: &Color) -> bool;
    }
}

Currently this results in generating something like the following, which does not compile.

namespace shared {
struct Color final {
  uint8_t r;
  uint8_t g;
  uint8_t b;

  bool is_white() const noexcept;
};
} // namespace shared

namespace rust_part {
bool Color::is_white() const noexcept {
  ...
}
} // namespace rust_part

cxxbridge/sources/demo/src/main.rs.cc:25:6: error: ‘Color’ has not been declared
   25 | bool Color::is_white() const noexcept {
      |      ^~~~~

I wouldn't be opposed to accepting that code and just making it put the member function in the struct's namespace all the time, regardless of namespace attribute on the member function.

But if someone feels strongly this should be an error (with a better message), that would be reasonable too.