This CL adds a copy of the internal error codes added to go/types in
https://golang.org/cl/264179. In a subsequent CL, we will use these to
extract the unexported error code from go/types errors via reflection.
A generated String method is added to provide a human-readable code for
the user.
For golang/go#42290
Change-Id: I280c9b111598426dce4eef38b9979919ed590068
Reviewed-on: https://go-review.googlesource.com/c/tools/+/267939
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>