Get out of the infinite loop by allowing to scan backwards when the current token is a comment.
Rearrange the logic for clarity and add a new unit test.
Fix the unit test issue and replace `while (true)` loop with clearer logic in `do … while` loop.
Merge pull request #9006 from adobe/rlim/hang-in-css-comment

Fix infinite loop by ensuring loop always makes progress when the current token is a comment.