Represent diagnostic side effects as dep nodes
This changes diagnostic to be tracked as a special dep node (`SideEffect`) instead of having a list of side effects associated with each dep node. `SideEffect` is always red and when forced, it emits the diagnostic and marks itself green. Each emitted diagnostic generates a new `SideEffect` with an unique dep node index.
Some implications of this:
- Diagnostic may now be emitted more than once as they can be emitted once when the `SideEffect` gets marked green and again if the task it depends on needs to be re-executed due to another node being red. It relies on deduplicating of diagnostics to avoid that.
- Anon tasks which emits diagnostics will no longer *incorrectly* be merged with other anon tasks.
- Reusing a CGU will now emit diagnostics from the task generating it.
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