transpile: const macros aren't translated with portable types

[kkysen]

This is an offshoot of #1320. After implementing the partial fix described in #1320 of inserting casts at const macro use sites, we still want to properly translate the macro with a portable type, as otherwise the type isn't portable once again, just like before #1266.

[fw-immunant]

This does seem to actually cause us to transpile some code to Rust that doesn't compile (minimized from python2's bytes_methods.c):

#include <string.h>

#define Py_MEMCPY memcpy
void f(void* x) {
	size_t len = 0;
	Py_MEMCPY(x, x, len);
}

transpiles to:

#![allow(
    dead_code,
    mutable_transmutes,
    non_camel_case_types,
    non_snake_case,
    non_upper_case_globals,
    unused_assignments,
    unused_mut
)]
extern "C" {
    fn memcpy(
        __dest: *mut std::ffi::c_void,
        __src: *const std::ffi::c_void,
        __n: size_t,
    ) -> *mut std::ffi::c_void;
}
pub type size_t = usize;
pub const Py_MEMCPY: unsafe extern "C" fn(
    *mut std::ffi::c_void,
    *const std::ffi::c_void,
    std::ffi::c_ulong,
) -> *mut std::ffi::c_void = unsafe { memcpy };
#[no_mangle]
pub unsafe extern "C" fn f(mut x: *mut std::ffi::c_void) {
    let mut len: size_t = 0 as size_t;
    memcpy(x, x, len);
}

Which results in:

error[E0308]: mismatched types
  --> memcpy_macro.rs:22:39
   |
22 | ) -> *mut std::ffi::c_void = unsafe { memcpy };
   |                                       ^^^^^^ expected fn pointer, found fn item
   |
   = note: expected fn pointer `unsafe extern "C" fn(_, _, u64) -> _`
                 found fn item `unsafe extern "C" fn(_, _, usize) -> _ {memcpy}`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.