lint: deny transmuting from immutable to mutable, since it's undefined behavior

[breaking-change] Technically breaking, since code that had been using
these transmutes before will no longer compile. However, it was
undefined behavior, so really, it's a good thing. Fixing your code would
require some re-working to use an UnsafeCell instead.

Closes #13146

src/test/compile-fail/transmute-imut-to-mut.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 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.
+
+// Tests that transmuting from &T to &mut T is Undefined Behavior.
+
+use std::mem::transmute;
+
+fn main() {
+    let _a: &mut u8 = unsafe { transmute(&1u8) };
+    //~^ ERROR mutating transmuted &mut T from &T may cause undefined behavior
+}
+
+