VSCode integration: the last "check" diagnostic lingers after getting fixed.

I'm using rules_rust in a cargoless setup with VSCode as the editor.

I generate rust-project.json using .vscode/tasks.json.

I also want "check" warnings such as "symbol unused". I've set that up with

```
{
"rust-analyzer.check.overrideCommand": [
"bazel",
"build",
"//...",
"--@rules_rust//:error_format=json",
]
}
```

This works well. However, when there is a single diagnostic and I fix it, that diagnostic keeps being reported. The only ways to make it go away are to:
* restart the editor
* add a new issue -- that will replace the lingering one, but that will linger in turn!

You can find a full repro + a video showcasing the issue at https://github.com/brunal/bazel-rust-analyzer-test/tree/master.

I wonder if, in the absence of issues, a "all is fine" message should be emitted.