[dev.typeparams] cmd/compile/internal/types2: delay interface check for type bounds

While at it, clean up code for collecting/declaring type parameters.

For #40789.

Change-Id: I0855137d5ee85c0ae2fa60d33b28c24a33132fbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/331690


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