Support controlling the severity of specific diagnostics and "diagnostic groups"

[d-ronnqvist]

Bug/issue #, if applicable: rdar://107343046

Summary

This adds support for controlling the severity of specific diagnostics and "diagnostic groups" using --Werror and --Wwarning flags. Those flag spellings were chosen because it's the spellings that Swift added in SE-0443.

Note

This draft PR is incomplete but sufficient to start the necessary Forum discussion about the broader design and user facing behaviors (flag names etc.).

I'm also planning to:

• Update the default diagnostic formatter to display the diagnostic group id (or diagnostic id as a fallback) within square braces, like Swift does.
• Update the SwiftDocC documentation (aimed at contributors to this repo) with guidance on how to name diagnostic IDs and how to group diagnostics since these identifiers would become user-facing.
• Go through all existing diagnostics to:
  • Organize them into meaningful groups
  • Remove the "org.swift.docc." prefix from their current identifiers—so that developers don't need to write that when controlling the severity of individual warnings
  • Rephrase the current identifiers to be more consistently named and to follow the new documentation about naming diagnostic identifiers.

Dependencies

None

Testing

• Add steps for how to test this once this feature is more complete.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[d-ronnqvist]

I started this Forum thread to discuss the user-facing aspects of this feature.