[analyzer] Track indices of arrays

Often times, when an ArraySubscriptExpr was reported as null or
undefined, the bug report was difficult to understand, because the
analyzer explained why arr[i] has that value, but didn't realize that in
fact i's value is very important as well. This patch fixes this by
tracking the indices of arrays.

Differential Revision: https://reviews.llvm.org/D63080

llvm-svn: 363510