If somebody writes
    
    func (s []int) _() {}
    
    we should report an error.  Before this patch we were silently
    accepting this code, because we didn't report any errors about the
    receiver if the method was a sink.
    
    This patch is unfortunately slightly complex to handle the case of
    
    func (x int) _() {}
    
    which we can only detect after define_global_names.
    
    This fixes blank1.go in the current gc testsuite.
    
    Reviewed-on: https://go-review.googlesource.com/43456

From-SVN: r248081