types2: provide error codes to error reporting

types2: provide error codes to error reporting

This CL adds a suitable error code to every error reporting,
matching what go/types already does.

For now, the error codes are not progagated through the API,
but eventually they will be available to clients.

Also, for now the errorcodes.go file is a 1:1 copy (with
adjusted package name) of go/types/errorcodes.go.
A subsequent CL will factor out this file.

In contrast to go/types, for errors due to incorrect Go version,
the error code is always _UnsupportedFeature. In go/types, the
error sometimes is related to the specific operation.

Change-Id: I18771bc3d00bbdbf6d705bf25f2aea3c3d977b1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/429355
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>