Miscompile (possible assertion failure) modifying nonmutating property on protocol

Users report their app is crashing due to a use-after-free when modifying a nonmutating property defined on a protocol extension. Test case project here:

https://github.com/iZettle/Flow/issues/34#issuecomment-429515408

They note that a snapshot compiler with asserts on hits an assertion failure:

https://github.com/iZettle/Flow/issues/34#issuecomment-424802558

The problem occurs when a property defined in a protocol or protocol extension with a nonmutating setter is mutated indirectly, as in:

import CoreGraphics

protocol SomeProtocol { }
class SomeClass: SomeProtocol { }

extension SomeProtocol {
    var someGetter: CGPoint {
        nonmutating set { _ = self }
        get { return .zero }
    }
}

SomeClass().someGetter.x = 42 // releases SomeClass() too early

The problem can be worked around by separating the `get` and `set` of the nonmutating property into their own statements:

let someObject = SomeClass()
var temp = someObject.someGetter // first get the property
temp.x = 42 // update the value
someObject.someGetter = temp // then set the property