[front end] Make code... objects usable as messages.

This is part of a series of CLs that will standardize CFE error
reporting to always use `codeFoo.withArguments(...)` when reporting
errors that take arguments and `codeFoo` when reporting errors that
don't take arguments, rather than `templateFoo.withArguments(...)`
when reporting errors that take arguments and `messageFoo` when
reporting errors that don't take arguments. This change will have two
advantages:

- It will lend greater consistency to the CFE codebase, by allowing
  the same `code...` objects to be used both to name error codes
  (e.g., in test expectations) and to report errors. This will allow
  everything associated with a certain error code to be found using a
  single invocation of "Find References" in the editor, rather than
  having to search separately for uses of the code and the message or
  template.

- It should hopefully make the experience of writing code that reports
  errors more pleasant, since it will no longer be necessary to look
  up an error to see whether it takes arguments before using it;
  instead, the developer will be able to type the name of the message
  `code...` declaration, and then use autocompletion to see whether
  `.withArguments(...)` is required.

In this CL, the code generator for error messages is changed so that
both `message...` and `code...` declarations have the type
`MessageCode` (and, indeed, refer to the same constant object). This
will allow uses of `message...` to be replaced with `code...`. In
follow-up CLs, the uses of `message...` will be changed to `code...`,
and the `message...` declarations will be removed.

Change-Id: I3ee1164678a25351c351bdb760274e84c21758ee
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443144
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441