This makes sure that its argument is marked live on entry.
We need its arg to be live so defers of KeepAlive get
scanned correctly by the GC.

Fixes #21402

Change-Id: I906813e433d0e9726ca46483723303338da5b4d7
Reviewed-on: https://go-review.googlesource.com/55150
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>