Prepare the Torque compiler to generate Kythe artifacts to be consumed
by CodeSearch.

Drive-by changes.
* Extend SourcePosition by an offset in the input string, as this is
  required by the Kythe graph.
* Correctly set missing identifier positions in Declarations.

Bug: v8:12261
Change-Id: Ida0a4a562c99f58ab924ddde36f3146f3d3fd415
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3181102
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#77099}