go/types: export TypeParam.Index and remove TypeParam._SetId

This change resolves a TODO regarding a couple uncertain APIs for
types.TypeParam. In the case of TypeParam._Index, we've decided it is
worth exporting. In the case of TypeParam._SetId, we've decided it is
unnecessary.

This aligns go/types with types2 (a doc comment in types2 is also
updated).

Updates #47916

Change-Id: I705e8b3437d014775c473e2f8be6f32b1540bb0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/354370


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