Ensure that array types with complicated length expressions are
    handled correctly by lowering expression types in the lowering pass.
    
    This required some adjustment of constant expression types to not
    report too many errors for circular constant expressions.  We now
    record error types in the Named_constant type.  If we find the
    circularity due to lowering the Named_constant, we use that location
    for the error message; this retains the error location we used to use.
    
    Fixes golang/go#23545
    
    Reviewed-on: https://go-review.googlesource.com/91035

From-SVN: r257250