[dev.typeparams] cmd/compile: allow embedding Type.Vargen into Sym.Name

Unified IR currently works by hoisting local type definitions to
package scope, which requires giving them a unique name. Its current
solution is to directly embed the ·N suffix in Sym.Name, rather than
set Type.Vargen. This CL extends types/fmt.go to support trimming this
suffix again when appropriate.

Longer term, I want to revisit this hack, but this seemed like the
least invasive solution while also handling generics and local types.

Change-Id: If99fcdcc1e19e37d5887de3b021c256a3fe46b98
Reviewed-on: https://go-review.googlesource.com/c/go/+/327052


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>