[dev.typeparams] cmd/compile/internal/types2: use comparable bit rather than ==() method

This removes the special "==" methods from comparable interfaces in
favor of a "comparable" flag in TypeSets indicating that the interface
is or embeds comparable. Fixes various related implementation
inaccuracies.

While at it, fix setup of the predeclared error and comparable
interface types by associating their respective type name objects
with them.

For #47411.

Change-Id: I409f880c8c8f2fe345621401267e4aaabd17124d
Reviewed-on: https://go-review.googlesource.com/c/go/+/337354


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