Merging r367802:

------------------------------------------------------------------------
r367802 | baloghadamsoftware | 2019-08-05 08:45:41 +0200 (Mon, 05 Aug 2019) | 6 lines

[Analyzer] Iterator Checkers - Fix for Crash on Iterator Differences

Iterators differences were mistakenly handled as random decrements which
causes an assertion. This patch fixes this.


------------------------------------------------------------------------

llvm-svn: 368427