check-on-save: print what is currently being built, not what has finished building

During check-on-save, RA shows the names of crates that have just finished building in the toolbar. However, that can be quite confusing: if crate A is right before crate B in the dependency chain, and B takes a long time, it'll show "A" for a long time, making it look like that's what's taking the time.

It'd be great if the display could be more like that of cargo, where it shows the crates currently being built instead of the ones that have just finished building. Assuming cargo emits a JSON message when a crate starts the build and when it ends, that should be possible by keeping around the state of which crates have started but not yet ended.

Start by locating the check-on-save build-status handling in rust-analyzer and the code that consumes Cargo JSON messages. Trace how completed crates are currently retained for the toolbar, then verify how start and end messages can represent crates still building. Done means the display reports currently building crates rather than the most recently completed crate; no file or test is named in the issue.