compiler: track //go:nointerface in export data

compiler: track //go:nointerface in export data

    The magic //go:nointerface comment, used for field tracking, was only
    implemented for conversions to interface types in the same package.
    Record it in the export data, so that it works as expected for types
    imported from a different package.

    Reviewed-on: https://go-review.googlesource.com/93075

From-SVN: r257540