cmd/compile: changed representation of typeparam bound in types1

Especially with typesets, we should be able to fully represent a
typeparam bound as just another type (actually an interface type).
Change the representation of a typeparam in types1 to include a bound,
which is just a type. Changed the signature for NewTypeParam() to take a
sym, and not a package, since we always set the sym (name) of the
typeparam when creating it. No need for an extra pkg field in Typeparam.

Also added index field in the types1 representation of typeparam. This
is especially needed to correctly export the typeparam, and re-import it
as a types2 type (which requires the index to be set correctly).

Change-Id: I50200e2489a97898c37d292b2bd025df790b0277
Reviewed-on: https://go-review.googlesource.com/c/go/+/319929


Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>