cmd/compile: unexport Type.Vargen

This field is only used outside of packages types in two places, and
they follow the same pattern. So this CL creates a Type.Setvargen
function that they can use instead, so that Type.Vargen can be
unexported.

A bit clumsy, but it works for now.

Change-Id: I7b4f33fac635e2464df2fbc0607ab40902f6f09f
Reviewed-on: https://go-review.googlesource.com/c/go/+/345469


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