[ObjC] avoid crashing when emitting synthesized getter/setter and ptrdiff_t is smaller than long

On targets where ptrdiff_t is smaller than long, clang crashes when emitting
synthesized getters/setters that call objc_[gs]etProperty.  Explicitly emit a
zext/trunc of the ivar offset value (which is defined to long) to ptrdiff_t,
which objc_[gs]etProperty takes.

Add a test using the AVR target, where ptrdiff_t is smaller than long. Test
failed previously and passes now.

Differential Revision: https://reviews.llvm.org/D112049