Depends on #4183

Add a, for now, optional title or message field to Fix that will be used to replace the DiagnosticKind::autofix_title.

The motivation is that a diagnostic can have multiple potential fixes. For example, the unused variable diagnostic can recommend to either:

• Prefix the variable with an underscore, to intentionally mark it as "this is ok"
• Remove the variable
• Add a noqa suppression comment.

Adding the title to the Fix instead of specifying it on the DiagnosticKind will allow us to model the different diagnostics.

It also has the advantage that the Rust compiler can enforce the requirement that each fixable DiagnosticKind has a message, because it is not stored as part of the Fix.

Tasks

• Add new title or message field to Fix (Option<String>).
• Change the constructor methods to take message: String as first argument, except for unspecified*, to avoid having to refactor all calls at once
• Serialize the title/message as part of the Fix in the JSON Emitter
• Change the MessageCodeFrame implementation to show DiagnosticKind::suggestion if present, or fall back to Fix::title if the Diagnostic has one Fix.