Incremental: Reset ckeys for anonymous structs and unions

[jerhard]

This PR adds code that resets the ckey for unchanged anonymous structs and unions. Previously, the cname for these structs and unions was already reset, but the ckey was not. This led to the issue in #678.

Background

Anonymous structs and unions (compounds) are provided with rather fragile names by Cil: The names of these compounds tend to change between program versions even when there is not semantic change relating to them. Therefore, there is special handling in CompareCIL that does not require that the names of anonymous compounds are the same as in the previous program version, as long as there is structural equality and they once appear in the same usage as in the previous program version. The cname of these anonymous compounds was reset to the one in the previous version.
This updating is done in CompareCIL, as opposed to updateCIL, because it is the easiest to also implement the requirement that the compound was used once in the same usage as in the previous program version.

However, the ckey, which is computed by Cil from the cname and the information whether the compinfo is referring to a struct or union, was not updated.

Fixes #678

[sim642]

I think another concern at the time I wrote #678 was how ckeys are generated non-sequentially in the first place.
With MaxIdUtil, we find maximum sequential IDs in order to renumber the new program's IDs to come after the previous maximums, ensuring no overlap. With ckeys this doesn't happen, so it's theoretically possible that due to a hash collision different structs in the old and new program by coincidence have the same ckey, causing issues down the line.

Although that is quite unlikely, so it might be fine to leave that for now.