[clang][tooling] Fix early termination when there are nested expansions

This also does some cleanups, I am happy to undo them (or send as
separate patches):
- Change the early exit to stop only once we hit an expansion inside the
  main file, to make sure we keep following the nested expansions.
- Add more tests to cover all the cases mentioned in the implementation
- Drop the adjustments for prev/next tokens. We do the final checks
  based on the expansion locations anyway, so any intermediate mapping
  was a no-op.

Differential Revision: https://reviews.llvm.org/D154335