compiler: implement type aliases

compiler: implement type aliases

    This is a start of implementing type aliases (`type T1 = T2`) in the
    Go frontend.  This is incomplete, in that the reflection information
    is not updated for an embedded type alias.  It is also not well
    tested.  Finally, the change to the language has not been approved.
    This should be regarded as preliminary work for experimental use.

    Update golang/go#18130.

    Reviewed-on: https://go-review.googlesource.com/35120

From-SVN: r244460