cmd/compile/internal/types2: use ExprString instead of syntax.String

cmd/compile/internal/types2: use ExprString instead of syntax.String

This further reduces the differences between go/types and types2.

Change-Id: I1426c2f7c58e2d1123d93f68fbdda01b0cc2d46e
Reviewed-on: https://go-review.googlesource.com/c/go/+/562836
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>