[analyzer] handle modification of vars inside an expr with comma operator

[analyzer] handle modification of vars inside an expr with comma operator

We should track mutation of a variable within a comma operator expression.
Current code in ExprMutationAnalyzer does not handle it.

This will handle cases like:

(a, b) ++ < == b is modified
(a, b) = c < == b is modifed

Patch by Djordje Todorovic.

Differential Revision: https://reviews.llvm.org/D58894

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@355605 91177308-0d34-0410-b5e6-96231b3b80d8