[ComInterfaceGenerator] Warn on visibility of interface and StringMarshallingCustomTy...

[ComInterfaceGenerator] Warn on visibility of interface and StringMarshallingCustomType (#87065)

The generated code is in file scoped classes, so the interface and StringMarshallingCustomType need to be at least internal visibility. These changes warn if that condition isn't met.

Originally, I expected to report a diagnostic, but still generate code for the interface. This required some changes to DiagnosticOr<T> to allow it to hold both a diagnostic and a value if the diagnostic didn't cause failure. In the end those changes weren't necessary, but I left them because I can see them being valuable at some point in the future.

Completes #84662