Coalesce all note-level diagnostics along with their preceding diagnostic into one message

[micjabbour]

According to the docs:

The NOTE level is used to staple more information onto previous diagnostics.

Currently, notes end up being discarded most of the time because they usually point to code that is not relevant to the change. It would be much better to have them appended to their preceding diagnostic since they usually carry some information that is helpful to interpret the warning.