[front_end] Rework diagnostic code generation.

This change reworks the logic for generating the files
`pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart` and
`pkg/front_end/lib/src/codes/cfe_codes_generated.dart`.

The following changes are made:

- The generated code is changed so that each `withArgumentsOld`
  function forwards to a corresponding `withArguments` function, which
  accepts named arguments rather than positional ones. In later CLs
  these new `withArguments` functions will be made available to call
  directly from CFE code.

- The code in the `withArguments` functions uses a uniform naming
  convention, where the parameter names match the parameter names
  specified in the error message templates, and the variables holding
  the result of string conversions are named by appending `_0` to the
  parameter names.*

  *In principle, variables suffixed with `_1`, `_2`, etc. might be
  generated in the circumstance where a single parameter gets
  converted to strings in multiple ways. But this doesn't happen in
  practice.

- The code in the `withArguments` functions converts arguments to
  strings in a uniform way, by calling either top level functions in
  `pkg/_fe_analyzer_shared/lib/src/messages/conversions.dart` or
  methods on the `TypeLabeler` class.

- The code generation logic in
  `pkg/front_end/tool/generate_messages_lib.dart` has been refactored
  considerably, with an eye toward allowing it to be further
  generalized in the future. In particular, in follow-up CLs I plan to
  allow each error message to define the precise set of placeholders
  it will use, and their types, as is done in the analyzer's
  `messages.yaml` file.

There is no change to functionality.

Change-Id: I6a6a69644e3bd3a5871bc4d156865f2ac499e8a6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/447823
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>