cmd/go: use a nil *Origin to represent "uncheckable"

Previously, we used the presence of individual origin fields
to decide whether an Origin could be checked for staleness,
with a nil Origin representing “use whatever you have”.

However, that turns out to be fairly bug-prone: if we forget
to populate an Origin somewhere, we end up with an incomplete
check instead of a non-reusable origin (see #61415, #61423).

As of CL 543155, the reusability check for a given query
now depends on what is needed by the query more than what
is populated in the origin. With that in place, we can simplify
the handling of the Origin struct by using a nil pointer
to represent inconsistent or unavailable origin data, and
otherwise always reporting whatever origin information we have
regardless of whether we expect it to be reused.

Updates #61415.
Updates #61423.

Change-Id: I97c51063d6c2afa394a05bf304a80c72c08f82cf
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/543216
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

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