[NFC] [C++20] [Modules] [P1689] [Scanner] Don't use thread pool in P1689 per file mode (#84285)

I suddenly found that the clang scan deps may use all concurrent threads
to scan the files. It makes sense in the batch mode. But in P1689 per
file mode, it simply wastes times and resources.

This patch itself should be a NFC patch. It simply moves codes.