Up-to-date check should consider AdditionalDependentFileTimes file creation/deletion

`AdditionalDependentFileTimes` includes files such as `.editorconfig` and `global.json`. Actually it includes all _potential_ locations that one of these files may exist at. Instances which are present on disk have a valid time stamp. Others have some filesystem-specific 'zero' value (maybe 1900-01-01?).

Changes to the set of paths for which we have a valid time stamp should mark the project dirty.

Unfortunately these files do not guarantee that outputs are updated, so we may end up in a case where a project remains out-of-date until an input actually changes.

[AB#1083953](https://devdiv.visualstudio.com/0bdbc590-a062-4c3f-b0f6-9383f67865ee/_workitems/edit/1083953)