cmd/compile/internal/types2: enable TestSelection API test

This test was never fully ported from go/types. Implement
a conversion function from syntax.Pos to string index so
that the test can be enabled again.

Also renamed the local variable syntax to segment to avoid
confusion with the syntax package.

Change-Id: I1b34e50ec138403798efb14c828545780f565507
Reviewed-on: https://go-review.googlesource.com/c/go/+/344253


Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>