Omit most builtins if bridge explicitly includes "rust/cxx.h"

[dtolnay]

Closes #703. Builtins are not dumped into the generated output file if either the #[cxx::bridge] module contains include!("rust/cxx.h") or the cxxbridge CLI is invoked to pass it as an additional include via --include rust/cxx.h.

Before:

$ echo '#[cxx::bridge] mod ffi { struct Test { s: String } }' | cxxbridge /dev/stdin
#include <array>
#include <cstdint>
#include <string>
#include <type_traits>

namespace rust {
inline namespace cxxbridge1 {
// #include "rust/cxx.h"

#ifndef CXXBRIDGE1_RUST_STRING
#define CXXBRIDGE1_RUST_STRING
class String final {
  ...
  ...
  ...
};
#endif // CXXBRIDGE1_RUST_STRING
} // namespace cxxbridge1
} // namespace rust

struct Test;

#ifndef CXXBRIDGE1_STRUCT_Test
#define CXXBRIDGE1_STRUCT_Test
struct Test final {
  ::rust::String s;

  using IsRelocatable = ::std::true_type;
};
#endif // CXXBRIDGE1_STRUCT_Test

After (--include rust/cxx.h):

$ echo '#[cxx::bridge] mod ffi { struct Test { s: String } }' | cxxbridge /dev/stdin --include rust/cxx.h
#include "rust/cxx.h"

struct Test;

#ifndef CXXBRIDGE1_STRUCT_Test
#define CXXBRIDGE1_STRUCT_Test
struct Test final {
  ::rust::String s;

  using IsRelocatable = ::std::true_type;
};
#endif // CXXBRIDGE1_STRUCT_Test

After (include!("rust/cxx.h");):

$ echo '#[cxx::bridge] mod ffi { extern "C++" { include!("rust/cxx.h"); } struct Test { s: String } }' | cxxbridge /dev/stdin
#include "rust/cxx.h"

struct Test;

#ifndef CXXBRIDGE1_STRUCT_Test
#define CXXBRIDGE1_STRUCT_Test
struct Test final {
  ::rust::String s;

  using IsRelocatable = ::std::true_type;
};
#endif // CXXBRIDGE1_STRUCT_Test