[dev.typeparams] cmd/compile: add ir.TypeNodeAt

This CL adds a variant of ir.TypeNode that allows specifying position
information. This shouldn't normally be needed/used, but it's
occasionally helpful for writing code that passes toolstash -cmp.

Change-Id: I2be5da0339fd1ec2bee01d6c5310bd2ef58c46b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/327049


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>