strings: move Clone to stringslite
Follow-up CL help package like unique use Clone.
Change-Id: Ie64adf7e1a331f47c3cfe178c368d72fc72493ff
GitHub-Last-Rev:
499476cc4acdf58ecf0fec9f7281bfb90edc7c82
GitHub-Pull-Request: golang/go#67106
Reviewed-on: https://go-review.googlesource.com/c/go/+/581956
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>