[clang-tidy] Improved accuracy of check list updater script

 - Added `FixItHint` comments to Check files for the script to mark those checks as offering fix-its when the fix-its are generated in another file.
 - Case insensitive file searching when looking for the file a checker code resides in.

Also regenerated the list, sphinx had no issue generating the docs after this.

Reviewed By: sylvestre.ledru

Differential Revision: https://reviews.llvm.org/D81932