[5.0] IRGen: Empty fields do have an entry in the field offset vector

[aschwaighofer]

This is an error introduced as the result of a refactoring a while ago
and means that we will store dependently typed stored properties at the
wrong offset in a generic struct if it has stored properties of empty
types before said property.

Explanation: We would compute the wrong offset for dependently typed properties inside generic structs if said property was preceded by a stored property of empty type.

public struct GenericStruct<T : Proto> {
  var empty: EmptyStruct = EmptyStruct()
  var dummy: Int = 0
  var opt: Optional<T> = nil
}

Scope: This was introduced as the result of a refactoring in the swift-4.1-branch timeframe.

Risk: Low. Single line change that only applies to empty types when we compute their offset in the field offset vector.

Testing: Swift CI Tests added.

rdar://36384871

[aschwaighofer]

Master PR: #13904

[aschwaighofer]

@swift-ci Please smoke test

[aschwaighofer]

@swift-ci Please smoke test

[aschwaighofer]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 0be6aad

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 0be6aad