Transparent compiler: test that TC reuses check results after project modifications

Since a lot of things is being redone in how results are cached and invalidated, we could also try to add some other improvements.

When changing a project by adding a new file or reordering existing ones, everything in this project is invalidated. We could, however, preserve results up to the point where there's an observable change for the analysis.

Consider this project:
```
1.fs
2.fs
3.fs
4.fs
Program.fs
```

Adding a file after 4.fs could, in theory, preserve cached results for almost the whole project, and the new file would be analyzed instantly, compared to having to analyze the first 4 files first, as things stand now. Reordering files like 3.fs and 4.fs could also allow keeping cached results for the first two files.

I think this would significantly improve user experience on bigger projects.