[dev.regabi] cmd/compile: add Interface, Signature, and Struct constructors

[dev.regabi] cmd/compile: add Interface, Signature, and Struct constructors

This CL adds the remaining constructors needed to abstract away
construction of Types, and updates the compiler to use them
throughout. There's now just a couple uses within test cases to
remove.

While at it, I also replace the Func.Outnamed field with a simple
helper function, which reduces the size of function types somewhat.

Passes toolstash/buildall.

Change-Id: If1aa1095c98ae34b00380d0b3531bd63c10ce885
Reviewed-on: https://go-review.googlesource.com/c/go/+/274713
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>