Modified the dead stores checker to...

1) Check if a dead store appears as a subexpression.  For such cases, we emit
   a verbose diagnostic so that users aren't confused.  This addresses:
   
   <rdar://problem/5968508> checker gives misleading report for dead store in loop
   
2) Don't emit a dead store warning when assigning a null value to a pointer.
   This is a common form of defensive programming.  We may wish to make
   this an option to the the checker one day.
   
   This addresses the feature request in the following email:
   
   http://lists.cs.uiuc.edu/pipermail/cfe-dev/2008-June/001978.html

llvm-svn: 52555