Fix a bug where the code would use subscript a std::vector with the size

of the vector. For a vector 'v', '&v[v.size()]' isn't a valid way to
compute a pointer one-past-the-end of the vector. Instead, write the
loop in terms of iterators and save the beginning iterator. Once we have
that we can compute the beginning pointer from the beginning iterator,
and compute the distance which we should increment the beginning pointer
by subtracting the iterators.

What might be simpler would be to convert the function accepting a raw
pointer for begin and end to accept iterators or a range or some other
construct, but I wanted to keep this to a minimal bug-fix change.

This fixes a crash on any debug STL implementation which checks for
indexing out of bounds.

llvm-svn: 231765