cmd/compile: switch to final unified IR export format

Now that there's a native go/types importer for unified IR, the
compiler no longer needs to stay backwards compatible with old iexport
importers.

This CL also updates the go/types and go/internal/gcimporter tests to
expect that the unified IR importer sets the receiver parameter type
to the underlying Interface type, rather than the Named type. This is
a temporary workaround until we make a decision on #49906.

Notably, this makes `GOEXPERIMENT=unified go test` work on generics
code without requiring `-vet=off` (because previously cmd/vet was
relying on unified IR's backwards-compatible iexport data, which
omitted generic types).

Change-Id: Iac7a2346bb7a91e6690fb2978fb702fadae5559d
Reviewed-on: https://go-review.googlesource.com/c/go/+/386004


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>