go/parser: simplify parsing of array or slice constraint types
Simplify the parsing of array or slice constraint types added in CL
359134, following the port in CL 360135.
Change-Id: Ia86d4b0149a222423d3b19623dd39d4aeb23857d
Reviewed-on: https://go-review.googlesource.com/c/go/+/360115
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>