Factor out bug report URL boilerplate into a utility function

[stmontgomery]

Many places in the codebase terminate the program (via fatalError, precondition, or assert) or throw SystemError and include a message asking the developer to file a bug report. The URL and surrounding boilerplate text for this request are duplicated verbatim across 18 call sites.

This PR factors this into a utility function to reduce repetition and make it easier to include these helpful messages consistently when adding new assertion sites in the future.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

Does it need to be a function call? Can't it just be an internal global and we interpolate it in?

[grynspan]

Or even better, just have SystemError and/or .system synthesize it?

[stmontgomery]

SystemError/.system is just one of the several ways the URL string is used. And the helper function also formats the "and include this information: ..." suffix when relevant, so I find that valuable.