cmd/compile/internal/types2: record error code in Error struct

cmd/compile/internal/types2: record error code in Error struct

go/types already does this, if slightly differently.

Change-Id: I9cf5f493714d865deec5ad23e2ee9b5c5d3f2f0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/475197
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>