[Sema] Patch to issue warning on comparing parameters with
nonnull attribute when comparison is always true/false. 
Original patch by Steven Wu. I have added extra code to prevent issuing of
warning  when the nonnull parameter is modified prior to the comparison.
This addition prevents false positives in the most obvious cases.
There may still be false positive warnings in some cases (as one of my tests
indicates), but benefit far outweighs such cases. rdar://18712242


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222264 91177308-0d34-0410-b5e6-96231b3b80d8