Point to enclosing block/fn on nested unsafe

When declaring nested unsafe blocks (`unsafe {unsafe {}}`) that trigger
the "unnecessary `unsafe` block" error, point out the enclosing `unsafe
block` or `unsafe fn` that makes it unnecessary.

src/test/compile-fail/lint-unused-unsafe.rs

@@ -1,66 +0,0 @@
-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// Exercise the unused_unsafe attribute in some positive and negative cases
-
-#![allow(dead_code)]
-#![deny(unused_unsafe)]
-
-
-mod foo {
-    extern {
-        pub fn bar();
-    }
-}
-
-fn callback<T, F>(_f: F) -> T where F: FnOnce() -> T { panic!() }
-unsafe fn unsf() {}
-
-fn bad1() { unsafe {} }                  //~ ERROR: unnecessary `unsafe` block
-fn bad2() { unsafe { bad1() } }          //~ ERROR: unnecessary `unsafe` block
-unsafe fn bad3() { unsafe {} }           //~ ERROR: unnecessary `unsafe` block
-fn bad4() { unsafe { callback(||{}) } }  //~ ERROR: unnecessary `unsafe` block
-unsafe fn bad5() { unsafe { unsf() } }   //~ ERROR: unnecessary `unsafe` block
-fn bad6() {
-    unsafe {                             // don't put the warning here
-        unsafe {                         //~ ERROR: unnecessary `unsafe` block
-            unsf()
-        }
-    }
-}
-unsafe fn bad7() {
-    unsafe {                             //~ ERROR: unnecessary `unsafe` block
-        unsafe {                         //~ ERROR: unnecessary `unsafe` block
-            unsf()
-        }
-    }
-}
-
-unsafe fn good0() { unsf() }
-fn good1() { unsafe { unsf() } }
-fn good2() {
-    /* bug uncovered when implementing warning about unused unsafe blocks. Be
-       sure that when purity is inherited that the source of the unsafe-ness
-       is tracked correctly */
-    unsafe {
-        unsafe fn what() -> Vec<String> { panic!() }
-
-        callback(|| {
-            what();
-        });
-    }
-}
-
-unsafe fn good3() { foo::bar() }
-fn good4() { unsafe { foo::bar() } }
-
-#[allow(unused_unsafe)] fn allowed() { unsafe {} }
-
-fn main() {}