Repr alignment attribute on shared structs

[dtolnay]

#[cxx::bridge]
mod ffi {
    #[repr(align(4))]
    struct S {
        b: [u8; 4],
    }
}

Expected C++ header:

struct alignas(4) S {
  std::array<std::uint8_t, 4> b;
};

• Rust reference: https://doc.rust-lang.org/1.51.0/reference/type-layout.html#the-alignment-modifiers
• C++ reference: https://en.cppreference.com/w/cpp/language/alignas

Currently the C++ code generator just errors on this attribute.

error[cxxbridge]: unrecognized repr
  ┌─ src/main.rs:3:12
  │
3 │     #[repr(align(4))]
  │            ^^^^^^^^ unrecognized repr