[dev.typeparams] cmd/compile: extract posMap from noder
This CL extracts the position mapping logic from noder and moves it
into a new posMap type, which can be more easily reused.
Passes toolstash -cmp.
Change-Id: I87dec3a3d27779c5bcc838f2e36c3aa8fabad155
Reviewed-on: https://go-review.googlesource.com/c/go/+/282916
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>