Format strings: suggest casts for NS(U)Integer and [SU]Int32 on Darwin.

These types are defined differently on 32-bit and 64-bit platforms, and
trying to offer a fixit for one platform would only mess up the format
string for the other. The Apple-recommended solution is to cast to a type
that is known to be large enough and always use that to print the value.

This should only have an impact on compile time if the format string is
incorrect; in cases where the format string matches the definition on the
current platform, no warning will be emitted.

<rdar://problem/9135072&12164284>

llvm-svn: 163266