[dev.typeparams] cmd/compile: refactor top-level typechecking in unified IR

This CL is a first step towards incremental typechecking during IR
construction within unified IR. Namely, all top-level declarations are
now typechecked as they're constructed, except for assignments (which
aren't really declarations anyway).

Change-Id: I65763a7659bf2e0f5e89dfe9e709d60e0fa4c631
Reviewed-on: https://go-review.googlesource.com/c/go/+/332097


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