Extract diagnostics about 'sealed when' to a separate one

^KT-59152

compiler/frontend/src/org/jetbrains/kotlin/cfg/WhenChecker.kt

@@ -480,7 +480,7 @@ object WhenChecker {
 
     fun checkSealedWhenIsReserved(sink: DiagnosticSink, element: PsiElement) {
         KtPsiUtil.getPreviousWord(element, "sealed")?.let {
-            sink.report(Errors.UNSUPPORTED.on(it, "sealed when"))
+            sink.report(Errors.UNSUPPORTED_SEALED_WHEN.on(it))
         }
     }
 }

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -78,6 +78,7 @@ public interface Errors {
     DiagnosticFactory0<KtElement> EXPLICIT_BACKING_FIELDS_UNSUPPORTED = DiagnosticFactory0.create(ERROR);
 
     DiagnosticFactory1<PsiElement, String> UNSUPPORTED_INHERITANCE_FROM_JAVA_MEMBER_REFERENCING_KOTLIN_FUNCTION = DiagnosticFactory1.create(ERROR);
+    DiagnosticFactory0<PsiElement> UNSUPPORTED_SEALED_WHEN = DiagnosticFactory0.create(ERROR);
 
     ////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -826,6 +826,7 @@ public class DefaultErrorMessages {
         MAP.put(EXPLICIT_BACKING_FIELDS_UNSUPPORTED, "Explicit backing field declarations are not supported in FE 1.0");
 
         MAP.put(UNSUPPORTED_INHERITANCE_FROM_JAVA_MEMBER_REFERENCING_KOTLIN_FUNCTION, "Inheritance of a Java member referencing 'kotlin.jvm.functions.FunctionN': {0} is unsupported", STRING);
+        MAP.put(UNSUPPORTED_SEALED_WHEN, "'sealed' in front of 'when' is reserved");
 
         MAP.put(EXCEPTION_FROM_ANALYZER, "Internal Error occurred while analyzing this expression:\n{0}", THROWABLE);
         MAP.put(MISSING_STDLIB, "{0}. Ensure you have the standard Kotlin library in dependencies", STRING);