Merge Folding into Attribute for efficiency

Since Folding and Attribute are basically the same internally, only one structure can be used to hold them both. The motivation for this change comes from @cullmann's comment #2 (comment 83751):

We have one TextLineData per line. If that shrinks, it has a large impact. e.g. one could think about packing the attributes + folding info into one vector. That will save the 2 pointers overhead for the second vector.

#2 (closed)