compiler: Make empty interface types for vars during parse time.

compiler: Make empty interface types for vars during parse time.

    When making the type for a variable with an empty interface type,
    the parser makes an interface type with a NULL method set and relies
    on later passes to correct this.

    For sink variables, which are ignored in later passes, the interface
    method table is never finalized and a compile time assertion is issued.
    Instead, the initial type generated by the parser should be the empty
    interface type.

    Fixes golang/go#11579.

    Reviewed-on: https://go-review.googlesource.com/12049

From-SVN: r226123