Auto merge of #24392 - seanmonstar:lint-transmute-mut, r=alexcrichton

The [UnsafeCell documentation says it is undefined behavior](http://doc.rust-lang.org/nightly/std/cell/struct.UnsafeCell.html), so people shouldn't do it.

This happened to catch one case in libstd that was doing this, and I switched that to use an UnsafeCell internally.

Closes #13146