[messages] Move shared diagnostics to pkg/_fe_analyzer_shared.

Moves all the diagnostics that are shared between the analyzer and the
CFE from `pkg/front_end/messages.yaml` to
`pkg/_fe_analyzer_shared/messages.yaml`.

This CL was generated by the following steps:

- Run the `move_shared_diagnostics.dart` script introduced in
  https://dart-review.googlesource.com/c/sdk/+/448606.

- Manually modify `pkg/front_end/messages.status` to reflect the
  diagnostics that have been moved.

Change-Id: I6a6a6964d618089f8fe8d447b4344800bad5d532
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448660
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441