cmd/compile/internal/types2: clearer object string for type parameters

- print "type parameter" rather than just "type"
- print the type bound rather than the underlying type
- added an object string test

Change-Id: Ibb572ff35b74f2c6ccb27641154f096770541130
Reviewed-on: https://go-review.googlesource.com/c/go/+/361401


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