Issue Description:

I have a property wrapper on an array property. For example:

@propertyWrapper
struct TestWrapper<ValueType> {
    var wrappedValue: ValueType
}
        
struct State {
    @TestWrapper
    var values: [String] = []
}

var state = State()
state.values = Array(repeating: "", count: 20000)
        
for i in 0..<20000 {
    state.values[i] = String(i)
}

Running this code is very slow (in both release and debug modes) as it causes something like 20,000 array copies to occur. If I remove the property wrapper, it runs very fast and has no array copying overhead. This is very unexpected behavior, IMO, and represents a rather nasty performance surprise when using property wrappers.

I originally posted about this on the forums here: https://forums.swift.org/t/property-wrapper-causing-excessive-array-copying/30637

Tested on Xcode 11.2.1 (11B53).