go/types, types2: move tparamIndex from unify.go into infer.go

go/types, types2: move tparamIndex from unify.go into infer.go

Minor code reorganization: the next version of unify.go doesn't
need this function anymore, so move it where it is still used.

Change-Id: I6744a2361b5dfe2564ec73787a7a110e85ac9f1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/463230
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>