internal/types: fix the iota value in error code declarations
The new "InvalidSyntaxTree" node in the error code declaration
inadvertently incremented the value of iota by 1. Fix this by moving it
to its own declaration.
Change-Id: I34b33a8caddbbb9e41f431321ec0e5863dc15055
Reviewed-on: https://go-review.googlesource.com/c/go/+/449475
Run-TryBot: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
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