[front_end] Use shared logic for interpreting messages.yaml file.

Reworks the logic in `pkg/front_end/tool/generate_messages_lib.dart`
to take advantage of shared functionality in
`pkg/analyzer_utilities/lib/messages.dart`. This reduces code
duplication between the CFE and analyzer, and should help pave the way
for sharing more diagnostic message generation logic over time.

The following declarations are moved into
`pkg/analyzer_utilities/lib/messages.dart` to avoid introducing a
circular dependency between `pkg/analyzer_utilities` and
`pkg/front_end`:

- `severityEnumNames`

- `_templateParameterNameToType`

- `Conversion`, `LabelerConversion`, `NumericConversion`, and
  `SimpleConversion`

- `ParsedPlaceholder`

Also, the enum `_TemplateParameterType` is removed in favor of the
shared enum `ErrorCodeParameterType`. To accommodate parameter types
that haven't yet been fully harmonized between the analyzer and CFE
representations, `ErrorCodeParameterType` is now capable of
representing CFE-only and analyzer-only types. In the long run I
expect to update the analyzer so that it fully supports all diagnostic
parameter types.

Change-Id: I6a6a6964928631a6f73a354f9f2d7275197f84b2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448249
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/messages/severity.dart

@@ -6,15 +6,6 @@ library _fe_analyzer_shared.messages.severity;
 
 enum CfeSeverity { context, error, ignored, internalProblem, warning, info }
 
-const Map<String, String> severityEnumNames = const <String, String>{
-  'CONTEXT': 'context',
-  'ERROR': 'error',
-  'IGNORED': 'ignored',
-  'INTERNAL_PROBLEM': 'internalProblem',
-  'WARNING': 'warning',
-  'INFO': 'info',
-};
-
 const Map<String, CfeSeverity> severityEnumValues = const <String, CfeSeverity>{
   'CONTEXT': CfeSeverity.context,
   'ERROR': CfeSeverity.error,