The UIR export data format can be reasonably expressed using EBNF.
The noder owns the definition of the export data format, so this
seems like a reasonable place to put this.

Change-Id: I0205ab29a3c5e57d670d7fd3164a8bd604ab8e59
Reviewed-on: https://go-review.googlesource.com/c/go/+/670616


Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Bypass: Mark Freeman <mark@golang.org>
Auto-Submit: Mark Freeman <mark@golang.org>