[messages] Standardize DiagnosticCode names to all lower case.

Changes the diagnostic message code generator so that the strings
passed to `DiagnosticCode.name` and `DiagnosticCode.uniqueName` are
always all lower case, regardless of how the name is specified in the
`messages.yaml` file.

This paves the way for a follow-up CL that will stnadardize the
capitalization of the message names in the `messages.yaml` files.

Change-Id: I6a6a6964d17d7d258ff2c1fad261072fa6f9a432
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/465720
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441