A new conversion warning for when an Objective-C object literal is implicitly
cast into a boolean true value.  This warning will catch code like:

if (@0) {}
if (@"foo") {}


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200356 91177308-0d34-0410-b5e6-96231b3b80d8