[analyzer] Quickfix: do not overflow in calculating offset in RegionManager

Addresses https://bugs.llvm.org/show_bug.cgi?id=36206

rdar://37159026

A proper fix would be much harder, and would involve changing the
appropriate code in ExprEngine to be aware of the size limitations of
the type used for addressing.

Differential Revision: https://reviews.llvm.org/D43218

llvm-svn: 326122