Use symbolic names XN_ROWID and XN_EXPR in place of the (-1) and (-2)

Use symbolic names XN_ROWID and XN_EXPR in place of the (-1) and (-2)
magic numbers in Index.aiColumn[].  Add asserts to help verify that
Index.aiColumn[] is always used correctly.  Fix one place in FK processing
where Index.aiColumn[] was not being used correctly.

FossilOrigin-Name: 7d272aa62cd4cbbf4b5d04e3b918de27671e8301