1. Add support for a simulated event queue to generate compilation events by computing declarations in the tree. Invoking GetDiagnostics to populate the event queue is one of the main reason for large allocations/VM in our perf tests. We can get back to using the compilation event queue when the compiler supports attaching an event queue to an existing compilation, without requiring a complete clone.

2. Move the cached compilation data (declarations, suppression state, etc.) out of a conditional weak table and strongly reference it in CompilationWithAnalyzers.

3. Reduce allocations in per-analyzer state maintenance by tracking analyzed declarations instead of pending declarations - latter can be very large for symbols with many declarations (e.g. global namespace).