Add support for overriding validation severity

severityOverride cannot set an event to SUPPRESSED. Use suppressions
for that. It cannot lower the severity of an event, only elevate. This
prevents a backdoor for disabling built-in validation or ignoring
ERROR events.

Closes #856

docs/source-2.0/spec/model-validation.rst

@@ -229,7 +229,7 @@ following properties:
       - Description
     * - id
       - ``string``
-      - **Required**. The validation event ID to suppress.
+      - **Required**. The hierarchical validation event ID to suppress.
     * - namespace
       - ``string``
       - **Required**. The validation event is only suppressed if it matches the
@@ -315,6 +315,57 @@ specific. Further, a suppression ID of "ABC" does not match an event ID of
       - No
 
 
+------------------
+Severity overrides
+------------------
+
+The ``severityOverrides`` metadata property is used to elevate the severity
+of validation events. This property contains an array of severity override
+objects that support the following properties:
+
+.. list-table::
+    :header-rows: 1
+    :widths: 20 20 60
+
+    * - Property
+      - Type
+      - Description
+    * - id
+      - ``string``
+      - **Required**. The hierarchical validation event ID to elevate.
+    * - namespace
+      - ``string``
+      - **Required**. The validation event is only elevated if it matches the
+        supplied namespace. A value of ``*`` can be provided to match any namespace.
+    * - severity
+      - ``string``
+      - Defines the :ref:`severity <severity-definition>` to elevate matching
+        events to. This value can only be set to ``WARNING`` or ``DANGER``.
+
+The following example elevates the events of ``SomeValidator`` to ``DANGER``
+in any namespace, and ``OtherValidator`` is elevated to ``WARNING`` but only
+for events emitted for shapes in the ``smithy.example`` namespace:
+
+.. code-block:: smithy
+
+    $version: "2"
+
+    metadata severityOverrides = [
+        {
+            namespace: "*"
+            id: "SomeValidator"
+            severity: "DANGER"
+        }
+        {
+            namespace: "smithy.example"
+            id: "OtherValidator"
+            severity: "WARNING"
+        }
+    ]
+
+    namespace smithy.example
+
+
 -------------------
 Built-in validators
 -------------------

smithy-model/src/main/java/software/amazon/smithy/model/loader/ModelAssembler.java

@@ -580,15 +580,18 @@ public ValidatedResult<Model> assemble() {
         }
 
         if (disableValidation) {
-            List<ValidationEventDecorator> decorators = validatorFactory.loadDecorators();
-            return new ValidatedResult<>(transformed, ModelValidator.decorateEvents(decorators, events));
-        }
-
-        try {
-            return validate(transformed, events);
-        } catch (SourceException e) {
-            events.add(ValidationEvent.fromSourceException(e));
+            ValidationEventDecorator decorator = ValidationEventDecorator.compose(validatorFactory.loadDecorators());
+            for (int idx = 0; idx < events.size(); idx++) {
+                events.set(idx, decorator.decorate(events.get(idx)));
+            }
             return new ValidatedResult<>(transformed, events);
+        } else {
+            try {
+                return validate(transformed, events);
+            } catch (SourceException e) {
+                events.add(ValidationEvent.fromSourceException(e));
+                return new ValidatedResult<>(transformed, events);
+            }
         }
     }
 
@@ -599,22 +602,22 @@ private void addMetadataToProcessor(Map<String, Node> metadataMap, LoadOperation
     }
 
     private ValidatedResult<Model> returnOnlyErrors(Model model, List<ValidationEvent> events) {
-        List<ValidationEventDecorator> decorators = validatorFactory.loadDecorators();
+        ValidationEventDecorator decorator = ValidationEventDecorator.compose(validatorFactory.loadDecorators());
         return new ValidatedResult<>(model, events.stream()
                                                   .filter(event -> event.getSeverity() == Severity.ERROR)
-                                                  .map(event -> ModelValidator.decorateEvent(decorators, event))
+                                                  .map(decorator::decorate)
                                                   .collect(Collectors.toList()));
     }
 
     private ValidatedResult<Model> validate(Model model, List<ValidationEvent> events) {
         // Validate the model based on the explicit validators and model metadata.
         // Note the ModelValidator handles emitting events to the validationEventListener.
-        List<ValidationEvent> mergedEvents = new ModelValidator()
+        List<ValidationEvent> mergedEvents = ModelValidator.builder()
                 .validators(validators)
                 .validatorFactory(validatorFactory)
                 .eventListener(validationEventListener)
                 .includeEvents(events)
-                .createValidator()
+                .build()
                 .validate(model);
 
         return new ValidatedResult<>(model, mergedEvents);