FIR IDE: Enforce an order on subclasses of sealed class

Currently the order is non-determinstic, causing non-determinstic
behavior in the IDE diagnostics. This change enforces an alphabetic
order.

idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/FirIdeSealedHierarchyProcessor.kt

@@ -84,6 +84,9 @@ class FirIdeSealedHierarchyProcessor(session: FirSession, scopeSession: ScopeSes
                 .mapNotNull { it.classIdIfNonLocal() }
                 .toMutableList()
 
+            // Enforce a deterministic order on the result.
+            subclasses.sortBy { it.toString() }
+
             data[regularClass] = subclasses
         }
 
@@ -98,4 +101,4 @@ class FirIdeSealedHierarchyProcessor(session: FirSession, scopeSession: ScopeSes
 private fun getPackageViaDirectoryService(ktClass: KtClass): PsiPackage? {
     val directory = ktClass.containingFile.containingDirectory ?: return null
     return JavaDirectoryService.getInstance().getPackage(directory)
-}
+}

idea/testData/checker/WhenNonExhaustive.fir.kt

@@ -49,7 +49,7 @@ sealed class Variant {
     object Another : Variant()
 }
 
-fun nonExhaustiveSealed(v: Variant) = <error descr="[NO_ELSE_IN_WHEN] 'when' expression must be exhaustive, add necessary 'is Something', 'Another' branches or 'else' branch instead">when</error>(v) {
+fun nonExhaustiveSealed(v: Variant) = <error descr="[NO_ELSE_IN_WHEN] 'when' expression must be exhaustive, add necessary 'Another', 'is Something' branches or 'else' branch instead">when</error>(v) {
     Variant.Singleton -> false
 }