cmd/compile: refactor symbol sorting logic

cmd/compile: refactor symbol sorting logic

This used to be duplicated in methcmp and siglt, because Sig used its
own representation for Syms. Instead, just use Syms, and add a
(*Sym).Less method that both methcmp and siglt can use.

Also, prune some impossible cases purportedly related to blank
methods: the Go spec disallows blank methods in interface method sets,
and addmethod drops blank methods without actually recording them in
the type's method set.

Passes toolstash-check.

Updates #24693.

Change-Id: I24e981659b68504d71518160486989a82505f513
Reviewed-on: https://go-review.googlesource.com/105936
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>