Bug #:

Submitted by:
Reviewed by:
Incorporate feedback from Chris (on the last check-in).
- added a shared hook for pre/post ++/-- CheckIncrementDecrementOperand().
- handle invalid arithmetic on incomplete types (void *, struct foo *, where the
body isn't in scope). Added a diagnostic for this case.
- added some comments and C99 annotations.
- Sema.h now includes Expr.h. I'd prefer not to, however it doesn't break
any layering.

llvm-svn: 39370