Rollup merge of #136817 - dianne:clean-and-comment-pat-migration, r=Nadrieril

Pattern Migration 2024: clean up and comment

This follows up on #136577 by moving the pattern migration logic to its own module, removing a bit of unnecessary complexity, and adding comments. Since there's quite a bit of pattern migration logic now (and potentially more in #136496), I think it makes sense to keep it separate from THIR construction, at least as much as is convenient.

r? ``@Nadrieril``

Author: matthiaskrgr