checker: set ast file fallbacks for vls

[dy-tea]

Fixes #25678.

Running the example code with v -vls-mode test.v still causes issues but in an unrelated way.

[spytheman]

Running the example code with v -vls-mode test.v still causes issues but in an unrelated way.

After #25595 , it needs also -check to prevent the cgen stage from running on incomplete data :-| .

[spytheman]

I think we should either make the builder stop right after the checker, in -vls-mode, or exit inside check_files .

[JalonSolov]

-vls-mode should never go through the rest of the compiler, so stopping it early would be a good thing.

[spytheman]

Thank you @dy-tea 🙇🏻 .