design: update type parameters design for type sets

For golang/go#43651
Fixes golang/go#45346

Change-Id: I2d946ba7c4b2c3a2b47c123434b45190627d5fdc
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/306689
Trust: Ian Lance Taylor <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: ianlancetaylor