This change redoes the fix for #16101 (CL 31092) in a different way by
making t.Clone return the template associated with the t.Name() while
allowing for the case that a template of the same name is define-d.

Fixes #17735.

Change-Id: I1e69672390a4c81aa611046a209008ae4a3bb723
Reviewed-on: https://go-review.googlesource.com/33210
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>