patch to issue warning on comparing parameters with
nonnull attribute when comparison is always
true/false. Patch by Steven Wu with few fixes and minor
refactoring and adding tests by me. rdar://18712242


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220496 91177308-0d34-0410-b5e6-96231b3b80d8