Improve 'unknown instrinsic' error message

If you had previously tried to get the ValueRef associated with an
intrinsic that hadn't been described in
`trans::context::declare_intrinsic()`, the compile would panic with
an empty message.

Now we print out details about the error in the panic message.

Author: Dylan McKay

src/librustc_trans/trans/context.rs

@@ -566,7 +566,7 @@ impl<'b, 'tcx> CrateContext<'b, 'tcx> {
         }
         match declare_intrinsic(self, key) {
             Some(v) => return v,
-            None => panic!()
+            None => panic!("unknown intrinsic '{}'", key)
         }
     }