incr.comp.: Allow for re-using object files that contain unused code.

[michaelwoerister]

Right now, incremental compilation can only re-use an object file if it is an exact match of that code that we want. In theory it would be possible though to keep using an object file that contains a superset of the code we need.

An implementation would probably need #48211 to be implemented first. CGU translation would also need to be de-querified since the re-use decision cannot be made by simply marking the CGU as green.

However, before putting effort into implementing this we should make sure that it would actually pay off. For this we need to collect some data on MonoItem invalidation (which again is covered by #48211).

[steveklabnik]

Triage: I don't know anything about the details here, but I assume this was never gotten to.