go/types,types2: allocate the used* maps in initFiles
As described in the associated comment, we need to reallocate usedVars
and usedPkgNames in initFiles, as they are nilled out at the end of
Checker.Files, which may be called multiple times.
Fixes #72122
Change-Id: I9f6eb86e072d9d43a8720f6a5e86d827de6006a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/655437
Auto-Submit: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>