[dev.regabi] cmd/compile: add NewBasic and cleanup universe

[dev.regabi] cmd/compile: add NewBasic and cleanup universe

This CL introduces types.NewBasic, for creating the predeclared
universal types, and reorganizes how the universe is initialized so
that all predeclared types are uniformly constructed.

There are now a bunch of Type fields that are no longer assigned
outside of the package, so this CL also introduces some new accessor
methods that a subsequent CL will mechanically introduce uses of.

Change-Id: Ie7996c3d5f1ca46cd5bfe45ecc91ebfa6a7b6c7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/274435
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>