K2. Minor. Rename FirConstExpression to FirLiteralExpression

FirConstExpression is usually confused with "constant" calculations,
while in fact, it just denotes a simple literal expression
and `1 + 1` isn't represented by a FirConstExpression.

^KT-64314 Fixed

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt

@@ -172,7 +172,7 @@ object FirSerializationPluginClassChecker : FirClassChecker(MppCheckerKind.Commo
     context(CheckerContext)
     private fun FirExpression.isEqualTo(other: FirExpression): Boolean {
         return when {
-            this is FirConstExpression<*> && other is FirConstExpression<*> -> kind == other.kind && value == other.value
+            this is FirLiteralExpression<*> && other is FirLiteralExpression<*> -> kind == other.kind && value == other.value
             this is FirGetClassCall && other is FirGetClassCall -> AbstractTypeChecker.equalTypes(
                 session.typeContext,
                 resolvedType,