Update rust-lang#1866 test case.

Author: jumbatm

src/test/ui/issues/issue-1866.rs

@@ -1,6 +1,7 @@
 // build-pass
 #![allow(dead_code)]
 #![allow(non_camel_case_types)]
+#![warn(clashing_extern_decl)]
 
 // pretty-expanded FIXME #23616
 
@@ -20,6 +21,7 @@ mod b {
         use super::rust_task;
         extern {
             pub fn rust_task_is_unwinding(rt: *const rust_task) -> bool;
+            //~^ WARN `rust_task_is_unwinding` redeclared with a different signature
         }
     }
 }

src/test/ui/issues/issue-1866.stderr

@@ -0,0 +1,17 @@
+warning: `rust_task_is_unwinding` redeclared with a different signature
+  --> $DIR/issue-1866.rs:23:13
+   |
+LL |             pub fn rust_task_is_unwinding(rt: *const rust_task) -> bool;
+   |             ------------------------------------------------------------ `rust_task_is_unwinding` previously declared here
+...
+LL |             pub fn rust_task_is_unwinding(rt: *const rust_task) -> bool;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+note: the lint level is defined here
+  --> $DIR/issue-1866.rs:4:9
+   |
+LL | #![warn(clashing_extern_decl)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+   = note: expected  `unsafe extern "C" fn(*const usize) -> bool`
+              found  `unsafe extern "C" fn(*const bool) -> bool`
+