Constructor reference missing from Index data with `S?()`

[keith]

With this code:

struct S {}

func foo(_ x: S) {
  _ = S?(x)
}

You get these index references:

/*
       constructor=init()=s:3foo1SVACycfc=definition, implicit, childOf
       struct=S=s:3foo1SV=definition
       v  */
struct S {}

/*
     function=foo(_:)=s:3fooAAyyAA1SVF=definition
     |     parameter=x=s:3fooAAyyAA1SVF1xL_ACvp=definition, childOf
     |     |  struct=S=s:3foo1SV=reference, containedBy
     v     v  v  */
func foo(_ x: S) {
  /*
      struct=S=s:3foo1SV=reference, containedBy
      v  */
  _ = S?(x)
}

Where there should be a reference to the Optional constructor being called, which you do get with an explicit .init:

/*
       constructor=init()=s:3foo1SVACycfc=definition, implicit, childOf
       struct=S=s:3foo1SV=definition
       v  */
struct S {}

/*
     function=foo(_:)=s:3fooAAyyAA1SVF=definition
     |     parameter=x=s:3fooAAyyAA1SVF1xL_ACvp=definition, childOf
     |     |  struct=S=s:3foo1SV=reference, containedBy
     v     v  v  */
func foo(_ x: S) {
  /*
      struct=S=s:3foo1SV=reference, containedBy
      v  */
  _ = S?(x)
  /*
      struct=S=s:3foo1SV=reference, containedBy
      |  constructor=init(_:)=s:SqyxSgxcfc=reference, call, calledBy, containedBy
      v  v  */
  _ = S?.init(x)

}

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