[dev.typeparams] cmd/compile/internal/importer: implement position reading

This CL finishes importReader.pos's stub implementation to actually
return syntax.Pos. New PosBase handling is analogous to
typecheck/iimport.go, except for using syntax.PosBase instead of
src.PosBase.

Change-Id: I7629f9f5e69a38ffc2eec772504d6fb2169e1f12
Reviewed-on: https://go-review.googlesource.com/c/go/+/320614


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>