[Observation] Reduce the scope of the observation shouldNotifyObservers to only apply to observation and not side effects of accessors like willSet or didSet

[phausler]

There is a degenerate case which a developer may have relied on a side effect of a didSet that has a repetitious value. This restores that case such that properties in the form:

var foo: Int { didSet { print("applied") } }

When setting foo = 3 and then setting it again foo = 3 now restores the behavior of printing "applied" twice. This still leaves observation informing a change once.

[phausler]

@swift-ci please smoke test

[phausler]

@swift-ci please smoke test