cmd/compile/internal/importer: support final unified IR export format
This updates the cmd/compile/internal/importer to natively support the
"final" unified IR export format. This is really just for unit tests
and symmetry with go/internal/gcimporter though, since
cmd/compile/internal/noder has its own types2.Importer.
Change-Id: I52fbb6134dbc0a903d62c1b04f95d33bd29e0414
Reviewed-on: https://go-review.googlesource.com/c/go/+/388617
Run-TryBot: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>