Translate #[must_use] attribute to C++17 [[nodiscard]]

[dtolnay]

References:

• https://doc.rust-lang.org/1.54.0/reference/attributes/diagnostics.html#the-must_use-attribute
• https://en.cppreference.com/w/cpp/language/attributes/nodiscard

#[cxx::bridge]
mod ffi {
    #[must_use]
    struct S {...}

    extern "Rust" {
        #[must_use = "..."]
        type T;

        #[must_use]
        fn f() -> i32;
    }
}

// generated c++

struct [[nodiscard]] S final {...};

struct [[nodiscard("...")]] T final : rust::Opaque {...};

[[nodiscard]] int32_t f() noexcept;