static_cast, reinterpret_cast, and const_cast can all be used in C++

integral constant expressions (for conversions to integer types,
naturally). I don't *think* that const_casts will ever get to this
point, but I also can't convince myself that they won't... so I've
taken the safe route and allowed the ICE checking code to look at
const_cast.

llvm-svn: 81453