[analyzer] Use a wider integer type for an array index.

Avoids unexpected overflows while performing pointer arithmetics in 64-bit code.
Moreover, neither PointerDiffType nor 'int' can be used as a common array index
type because arrays may have size (and indexes) more than PTRDIFF_MAX but less
than SIZE_MAX.

Patch by Aleksei Sidorin!

Differential Revision: http://reviews.llvm.org/D16063

llvm-svn: 259345