[dev.regabi] cmd/compile: cleanup export code further

This CL rips off a number of toolstash bandages:

- Fixes position information for string concatenation.

- Adds position information for struct literal fields.

- Removes unnecessary exprsOrNil calls or replaces them with plain
  expr calls when possible.

- Reorders conversion expressions to put type first, which matches
  source order and also the order the importer needs for calling the
  ConvExpr constructor.

Change-Id: I44cdc6035540d9ecefd9c1bcd92b8711d6ed813c
Reviewed-on: https://go-review.googlesource.com/c/go/+/279957


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>