Correctly handle skewed streams in drop_front() method.

When calling BinaryStreamArray::drop_front(), if the stream
is skewed it means we must never drop the first bytes of the
stream since offsets which occur in records assume the existence
of those bytes.  So if we want to skip the first record in a
stream, then what we really want to do is just set the begin
pointer to the next record.  But we shouldn't actually remove
those bytes from the underlying view of the data.

llvm-svn: 349066