cmd/compile/internal/abi: stop using types.Func

cmd/compile/internal/abi: stop using types.Func

This is supposed to be an internal type within package types. At least
for now, users of the types package should stick to the types.Type
APIs as much as possible.

This CL also unexports FuncType and a few others to prevent
backsliding.

Change-Id: I053fc115a5e6a57c148c8149851a45114756072f
Reviewed-on: https://go-review.googlesource.com/c/go/+/521255
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>