Fix tracking for generated IDs for most XML nodes

- Delete the 16-bit constant for source ID masks, since libxml only
  provides 32-bit fields now; 11 bits for the source ID is almost
  certainly too few for even medium-sized documents. Increasing the
  extra field to 32 bits on _xmlNode fixes a bug where the extra field
  was shifted too far for flag extraction, causing the flags to always
  be treated as if they were unset on _xmlNode.
- Track generated IDs for XML nodes, et cetera so that subsequent
  calls to generate-id() return the same value. This is the core of the
  fix.
- Simplify clearing the source ID information after the transform is
  complete: it only requires zeroing out the extra field now, since
  libxslt no longer tries to reuse random libxml fields in clever ways
  for this particular purpose
- Introduce a new helper for clearing the extra data off source nodes.
  A previous attempt cleared the extra data by simply setting it 0.
  However, libxslt has a tendency to pass around a number of distinct
  structs (e.g. _xmlDoc, _xmlAttr) as a generic _xmlNode. While these
  structs have a number of fields in common, extra is not one of them,
  and this clobbered random fields on _xmlDoc.

Bug: 417859451
Bug: 417984350
Change-Id: I39dd9b5a417b3f72772d104a6c495b59599bd2e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6553256
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1461556}