Add diagnostic info message about unused conditional registrations

[dotnetjunkie]

Analyzer.Analyze returns, beside diagnostic warnings, diagnostic info messages as well.

It can be useful to add an additional info message that notes the case where a conditional registration is never used. Conditional registrations will be picked in order and can be selected using a predicate, but it can be easy to miss the fact that a registration is always skipped.

As this might be intentional behavior, or specific to a particular setup, it is unwise to implement this as a warning, as that would cause an exception when Verify() is called. An info message is, therefore, much more suited.