This commit changes fixit messages from a question/suggestion to an
imperative message for protocol conformances and switch-case. Addresses
https://github.com/apple/swift/issues/67510.
1. Make sure the actions taken by fixits are reflected in diagnostics messages.
2. Issue missing cases diagnostics at the start of the switch statement instead of its end.
3. Use <#code#> instead of <#Code#> in the stub.