cmd/compile/internal/types2: review of example_test.go

The changes between (equivalent, and reviewed) go/types/example_test.go
and example_test.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker.

The primary differences to go/types/example_test.go are:
- use of syntax instead of go/ast package
- no ExampleMethodSet test (types2 doesn't have MethodSet)
- some code in ExampleInfo is disabled due to less precise
  position information provided by the syntax tree

Change-Id: I035284357acc8ecb7849022b5a9d873ae2235987
Reviewed-on: https://go-review.googlesource.com/c/go/+/304249


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>