[CSDiagnostics] Add an error message for pack expansion expressions over packs that don't have the same shape.

[hborla]

For example:

func sameShapeDiagnostics<T..., U...>(t: T..., u: U...) {
  _ = (each t, each u)... // 't' and 'u' do not have the same shape
}

A ShapeOf constraint breaks down into a Bind. Previously if that failed, the constraint system would report type of expression is ambiguous without more context. Now, it reports pack expansion requires that 'U' and 'T' have the same shape.

[hborla]

@swift-ci please smoke test