Adjust the encoding m_pos uses when pointing past the last element

Summary: This diff changes the way arrays encode m_pos when it points past the last
element. Before, ArrayData::invalid_index (-1) was used. This diff changes
it to uses a non-negative value that is greater than the position of the
last element (if there is a last element). This allows PackedArrays and
MixedArrays to set m_pos to 0 initially, and then they never need to update
m_pos when a new element is added.

Reviewed By: @edwinsmith

Differential Revision: D1430872