PR5683: Issue a warning when subtracting pointers to types of zero size, and
treat such subtractions as being non-constant. Patch by Serge Pavlov! With a
few tweaks by me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190439 91177308-0d34-0410-b5e6-96231b3b80d8