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/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
 }