swift234

[Diagnostics] Refactor and clarify diagnostic behavior rules.

Restores StoredDiagnosticInfo, which is useful to help distinguish
when the user explicitly modifies the behavior of a diagnostic vs
we're just picking up the default kind.

Adds some clarifying comments, and lays out the suppression workflow,
whereby different types of suppression (per-diagnostic, per-category,
etc) have different precedence levels.