compiler: report correct errors for sink methods

compiler: report correct errors for sink methods

    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