Handle never type in return position consistently.

Fixes #779

tests/rust/function_noreturn.rs

@@ -1,9 +1,14 @@
 #[no_mangle]
-pub extern fn loop_forever() -> ! {
+pub extern "C" fn loop_forever() -> ! {
     loop {}
 }
 
 #[no_mangle]
-pub extern fn normal_return() -> u8 {
+pub extern "C" fn normal_return(arg: Example, other: extern "C" fn(u8) -> !) -> u8 {
     0
 }
+
+#[repr(C)]
+pub struct Example {
+    pub f: extern "C" fn(usize, usize) -> !,
+}