Update rust-lang#5791 test case.

Author: jumbatm

src/test/ui/issues/issue-5791.rs

@@ -1,11 +1,13 @@
 // run-pass
 #![allow(dead_code)]
+#![warn(clashing_extern_decl)]
 // pretty-expanded FIXME #23616
 
 extern {
     #[link_name = "malloc"]
     fn malloc1(len: i32) -> *const u8;
     #[link_name = "malloc"]
+    //~^ WARN `malloc2` redeclares `malloc` with a different signature
     fn malloc2(len: i32, foo: i32) -> *const u8;
 }
 

src/test/ui/issues/issue-5791.stderr

@@ -0,0 +1,19 @@
+warning: `malloc2` redeclares `malloc` with a different signature
+  --> $DIR/issue-5791.rs:9:5
+   |
+LL | /     #[link_name = "malloc"]
+LL | |     fn malloc1(len: i32) -> *const u8;
+   | |______________________________________- `malloc` previously declared here
+LL | /     #[link_name = "malloc"]
+LL | |
+LL | |     fn malloc2(len: i32, foo: i32) -> *const u8;
+   | |________________________________________________^ this signature doesn't match the previous declaration
+   |
+note: the lint level is defined here
+  --> $DIR/issue-5791.rs:3:9
+   |
+LL | #![warn(clashing_extern_decl)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+   = note: expected  `unsafe extern "C" fn(i32) -> *const u8`
+              found  `unsafe extern "C" fn(i32, i32) -> *const u8`
+