[messages] Change diagnostic codes to lower case in error_fix_status.yaml.

This paves the way for a follow-up CL that will standardize all
diagnostic names to `lower_snake_case` conventions.

For now, the case differences between the entries in
`error_fix_status.yaml` and the actual diagnostic code names are
accounted for by adding some calls to `.toLowerCase()` to
`verify_error_fix_status.dart`.

Change-Id: I6a6a6964b0eacf50b11f302ab511fba149b1ac07
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/465981
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: stereotype441