cmd/compile: do not declare func nodes

The primary responsibility of declare() to associate a symbol (Sym) with
a declaration (Node), so "oldname" will work. Function literals are
anonymous, so their symbols does not need to be declared.

Passes toolstash-check.

Change-Id: I739b1054e3953e85fbd74a99148b9cfd7e5a57eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/249078


Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>