go/types: match types2 errors for missing index expressions
Use "middle" and "final" rather than "2nd" and "3rd" in error messages
for invalid slice expressions. This is the original compiler error
message and many tests check for this specific message.
For #54511.
Change-Id: I86eb739aa7218b7f393fab1ab402732cb9e9a1f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/424906 Reviewed-by: Robert Findley <rfindley@google.com> Reviewed-by: Alan Donovan <adonovan@google.com> Reviewed-by: Robert Griesemer <gri@google.com>