Warn for non-nullable store-generated bool properties

[ajcvickers]

For bool properties the only valid values are true and false. If the value is false, then it is the CLR default, which triggers store generation. Therefore it is impossible to insert false values. This is unlikely to be the desired behavior. The usual fix for this is to make the property nullable.

For enums the situation is slightly different. The CLR default is always zero, which might also be a defined enum value, but also might not be. Even if it is a valid enum value it might be defined as something like "Default" possibly indicating that value generation should happen.

[ajcvickers]

Triage decision: warn only for bools.