make #[unwind] attribute specify expectations more clearly

You can now choose between the following:

- `#[unwind(allowed)]`
- `#[unwind(aborts)]`

Per rust-lang/rust#48251, the default is `#[unwind(allowed)]`, though
I think we should change this eventually.

src/test/codegen/extern-functions.rs

@@ -19,7 +19,7 @@ extern {
     fn extern_fn();
 // CHECK-NOT: Function Attrs: nounwind
 // CHECK: declare void @unwinding_extern_fn
-    #[unwind]
+    #[unwind(allowed)]
     fn unwinding_extern_fn();
 }
 

src/test/run-pass/abort-on-c-abi.rs

@@ -19,6 +19,7 @@ use std::io::prelude::*;
 use std::io;
 use std::process::{Command, Stdio};
 
+#[unwind(aborts)]
 extern "C" fn panic_in_ffi() {
     panic!("Test");
 }