Issue Description:

For the following code:

struct S : Decodable {
  @available(*, unavailable)
  var i: Int
}

we currently produce the location-less diagnostic:

<unknown>:0: error: 'i' is unavailable
/Users/hamishknight/Desktop/Stochastic Projects/newnew/newnew/main.swift:160:7: note: 'i' has been explicitly marked unavailable here
  var i: Int
      ^

The same also applies to Encodable.

Either we should refuse to synthesise a conformance or we should ignore the availability of i within the synthesised body (like we do for the memberwise initialiser, as we emit its body in SILGen).