Fix objcImpl bug causing invalid @_hasStorage attributes in module interfaces

[beccadax]

Consider this code:

@objc @implementation extension MyObjCClass {
    public final var myProperty: Int {
        didSet { print("myProperty changed") }
    }
}

Because the var is public final, it needs to be printed into the module interface as part of an ordinary (non-objcImpl) extension. However, because the didSet observer is present, Swift would print a @_hasStorage attribute:

extension MyObjCClass {
    @_hasStorage public final var myProperty: Int {
        get
        set
    }
}

This is bad, of course, because stored properties are not allowed in ordinary extensions. But it's worse than that: because @_hasStorage is not particularly well validated, the compiler would not emit a diagnostic about this problem—it would simply crash. Often at a fairly arbitrary point if asserts were disabled.

This PR fixes this bug from basically all possible angles:

• Diagnoses uses of @_hasStorage in invalid contexts.
• Prevents emission of @_hasStorage for vars in module interfaces.
• Ignores invalid @_hasStorage attributes in existing module interfaces.

Fixes rdar://144811653.

[beccadax]

@swift-ci please test

[beccadax]

@swift-ci please smoke test Windows platform