[K2/N] KT-56190 Serialize simple const property initializers

Merge-request: KT-MR-8623
Merged-by: Vladimir Sukharev <Vladimir.Sukharev@jetbrains.com>

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirAnnotationSerializer.kt

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir.serialization
 
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.expressions.FirAnnotation
+import org.jetbrains.kotlin.fir.expressions.FirConstExpression
 import org.jetbrains.kotlin.fir.expressions.FirExpression
 import org.jetbrains.kotlin.fir.resolve.toSymbol
 import org.jetbrains.kotlin.fir.serialization.constant.ConstantValue
@@ -44,4 +45,7 @@ class FirAnnotationSerializer(private val session: FirSession, internal val stri
                 FirAnnotationArgumentVisitorData(this@FirAnnotationSerializer, this)
             )
         }
+
+    fun valueProto(constExpression: FirConstExpression<*>): ProtoBuf.Annotation.Argument.Value.Builder? =
+        constExpression.toConstantValue(session)?.let { valueProto(it) }
 }

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt

@@ -373,12 +373,15 @@ class FirElementSerializer private constructor(
             }
         }
 
+        val hasConstant = !property.isVar && property.initializer?.let {
+            it is FirConstExpression<*> && it.value != null
+        } ?: false
         val flags = Flags.getPropertyFlags(
             hasAnnotations,
             ProtoEnumFlags.visibility(normalizeVisibility(property)),
             ProtoEnumFlags.modality(modality),
             ProtoBuf.MemberKind.DECLARATION,
-            property.isVar, hasGetter, hasSetter, property.isConst, property.isConst, property.isLateInit,
+            property.isVar, hasGetter, hasSetter, hasConstant, property.isConst, property.isLateInit,
             property.isExternal, property.delegateFieldSymbol != null, property.isExpect
         )
         if (flags != builder.flags) {