[analyzer] Improve pointer arithmetic checker.

[analyzer] Improve pointer arithmetic checker.

This patch is intended to improve pointer arithmetic checker.
From now on it only warns when the pointer arithmetic is likely to cause an
error. For example when the pointer points to a single object, or an array of
derived types.

Differential Revision: http://reviews.llvm.org/D14203

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261632 91177308-0d34-0410-b5e6-96231b3b80d8