compiler: update iota handling, fix using iota in array length

compiler: update iota handling, fix using iota in array length

    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