This CL simplifies and removes some old noding code, which isn't
necessary any more.

Most notably, we no longer need separate posMaps for each noder,
because noders are only used for parsing now. Before we started using
types2, noders were also responsible for constructed (untyped) IR, so
posMaps were necessary to translate syntax.Pos into src.XPos.

Change-Id: Ic761abcd727f5ecefc71b611635a0f5b088c941f
Reviewed-on: https://go-review.googlesource.com/c/go/+/463738


Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>