DiagnosticRecord should support ToString()

Script Analyzer records use the default ToString(), which merely displays the type.

This means that to test the result of a script analyzer record with a regex, one needs to dig into subproperties.

It also means that records will not render ideally when stringified.

Proposed technical implementation details (optional)

[Microsoft.Windows.PowerShell.ScriptAnalyzer.Generic.DiagnosticRecord] should override .ToString() method to output the message.