Issue Description:

The following code always prints false, even though the capturing appears as if it should be happening after the call to isKnownUniquelyReferenced:

class Foo {}

struct Bar {
    
    var foo = Foo()
    
    public mutating func bar() {
        
        print(isKnownUniquelyReferenced(&foo))
        
        func nestedFunc() {
            _ = self // capture self
        }
        nestedFunc()
    }
}

var b = Bar()
b.bar() // false ?!

From what I can tell, self is getting alloc_box'ed too early (really shouldn't be boxed at all given that Swift prevents nestedFunc from escaping). This can break implementations that utilise isKnownUniquelyReferenced to optimise copy-on-write, in a rather non-obvious manner.