[analyzer] RetainCount: Add support for OSRequiredCast().

It's a new API for custom RTTI in Apple IOKit/DriverKit framework that is
similar to OSDynamicCast() that's already supported, but crashes instead of
returning null (and therefore causing UB when the cast fails unexpectedly).
Kind of like cast_or_null<> as opposed to dyn_cast_or_null<> in LLVM's RTTI.

Historically, RetainCountChecker was responsible for modeling OSDynamicCast.
This is simply an extension of the same functionality.

Differential Revision: https://reviews.llvm.org/D63117

llvm-svn: 363891