CL 71750 changed the definition of how iota works.  This patch updates
    gccgo for the new definition.
    
    We've been mishandling iota appearing in a type that appears in a
    const expression, as in `c = len([iota]int{})`.  Correct that by copying
    type expressions when we copy an expression.  For simplicity only copy
    when it can change the size of a type, as that is the only case where
    iota in a type can affect the value of a constant (I think).  This is
    still a bunch of changes, but almost all boilerplate.
    
    Fixes golang/go#22341
    
    Reviewed-on: https://go-review.googlesource.com/91475

From-SVN: r257379