[dev.typeparams] cmd/compile/internal/types2: translate syntax to token constants via tables

This makes the respective files match the respective go/types files
a tad more.

Change-Id: Ie555e18ed23c493379a1e56b96276867190106f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/287492


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>