Auto merge of #122156 - Zoxc:side-effect-dep-node, r=oli-obk

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.

compiler/rustc_session/src/options.rs

@@ -2525,7 +2525,7 @@ written to standard error output)"),
         "for every macro invocation, print its name and arguments (default: no)"),
     track_diagnostics: bool = (false, parse_bool, [UNTRACKED],
         "tracks where in rustc a diagnostic was emitted"),
-    // Diagnostics are considered side-effects of a query (see `QuerySideEffects`) and are saved
+    // Diagnostics are considered side-effects of a query (see `QuerySideEffect`) and are saved
     // alongside query results and changes to translation options can affect diagnostics - so
     // translation options should be tracked.
     translate_additional_ftl: Option<PathBuf> = (None, parse_opt_pathbuf, [TRACKED],