go/types: fix version downgrade bug without Config.GoVersion

go/types: fix version downgrade bug without Config.GoVersion

The gVisor team reported a regression in their checkers,
which don't set Config.GoVersion, processing files that say
//go:build go1.13 but still use 'any' (which happened in Go 1.18).
That situation should continue to work, since it worked before,
so add a special case for not knowing the GoVersion.

Change-Id: I8820d8ccbdf76d304e2c7e45f6aaa993ff3d16a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/486398
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>