reflect: keep pointer in aggregate-typed args live in Call

When register ABI is used, reflect.Value.Call prepares the call
arguments in a memory representation of the argument registers.
It has special handling to keep the pointers in arguments live.
Currently, this handles pointer-typed arguments. But when an
argument is an aggregate-type that contains pointers and passed
in registers, it currently doesn't keep the pointers live. Do
so in this CL.

May fix #49363.

Change-Id: Ic6a0c5fdf9375ef02f7c03fbe9345e2e98c9353d
Reviewed-on: https://go-review.googlesource.com/c/go/+/363358


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>