go/types, types2: collect per-file Go version in Info.FileVersions

go/types, types2: collect per-file Go version in Info.FileVersions

The go/types changes are matching but the API changes are hidden
for now, pending acceptance of a respective proposal.

Change-Id: I2e38ff215ddbdcf93f182d3e70c03802d3ca4338
Reviewed-on: https://go-review.googlesource.com/c/go/+/515135
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>