cmd/compile/internal/syntax: fix parsing of array or slice constraint types

cmd/compile/internal/syntax: fix parsing of array or slice constraint types

This is a port of the idea used in CL 359134 from go/parser to syntax,
with adjustments due to the slightly different structure of the two
parsers, and some refactoring to simplify the logic.

Fixes #49175.

Change-Id: Ib4955bde708f2b08345f35523e6094c03ab3076c
Reviewed-on: https://go-review.googlesource.com/c/go/+/360135
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>