When the compiler failed to evaluate a numeric constant because
    because it was erroneous, there was no way for parent nodes to
    discover the error and lower themselves into error nodes.  This patch
    now uses the NC_INVALID enumerator to mark numeric constants with a
    known, reported error.
    
    Fixes golang/go#11541.
    
    Reviewed-on: https://go-review.googlesource.com/13904

From-SVN: r227420