Revert r225053: Add an ArrayRef upcasting constructor from ArrayRef<U*> ->...

Revert r225053: Add an ArrayRef upcasting constructor from ArrayRef<U*> -> ArrayRef<T*> where T is a base of U.

This appears to have broken at least the windows build bots due to
compile errors in the predicate that didn't simply supress the overload.
I'm not sure what the fix is, and the bots have been broken for a long
time now so I'm just reverting until Michael can figure out a fix.

llvm-svn: 225064