go/types, types2: avoid spurious "undefined" errors" for invalid identifiers

go/types, types2: avoid spurious "undefined" errors" for invalid identifiers

The syntax parser complains about invalid identifiers.
Don't report a typechecker error when such an identifier
cannot be found in the current scope.

For now add a local test for types2 only because the
go/parser behaves differently than the syntax parser
which leads to slightly different error positions.

Fixes #68183.

Change-Id: Idbfe62fafcd704886069182744ec5e6b37ffc4e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/602476
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>