[FIR] Make synthetic FunctionN declarations MustUse by default

Since they are not declared in stdlib sources, we cannot compile them
with the MustUse flag normally as any other stdlib builtin.
To handle them correctly in UnusedReturnValueChecker, their status
should be set where they are created.

#KT-71195

Author: sandwwraith

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirUnusedReturnValueChecker.kt

@@ -171,9 +171,6 @@ private fun ConeKotlinType.isIgnorable(): Boolean {
 private fun FirCallableSymbol<*>.isExcluded(session: FirSession): Boolean = session.mustUseReturnValueStatusComponent.hasIgnorableLikeAnnotation(resolvedAnnotationClassIds)
 
 private fun FirCallableSymbol<*>.isSubjectToCheck(): Boolean {
-    // TODO KT-71195 : FunctionN seems to be a synthetic class, even recompiling stdlib in FULL mode
-    //  does not make it checked. Need to investigate another approach.
-    if (this.callableId?.packageName?.asString() == "kotlin") return this.origin !is FirDeclarationOrigin.Enhancement
     callableId?.ifTypealiasedJvmCollection { return it }
 
 

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/providers/impl/FirExtensionSyntheticFunctionInterfaceProvider.kt

@@ -36,6 +36,7 @@ import org.jetbrains.kotlin.fir.toFirResolvedTypeRef
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.fir.types.impl.ConeTypeParameterTypeImpl
 import org.jetbrains.kotlin.name.*
+import org.jetbrains.kotlin.resolve.ReturnValueStatus
 import org.jetbrains.kotlin.types.Variance
 import org.jetbrains.kotlin.util.OperatorNameConventions
 import java.util.concurrent.ConcurrentHashMap
@@ -229,6 +230,7 @@ abstract class FirSyntheticFunctionInterfaceProviderBase(
                         isOperator = true
                         isSuspend = kind.isSuspendOrKSuspendFunction
                         hasStableParameterNames = false
+                        returnValueStatus = ReturnValueStatus.MustUse
                     }
                     val typeArguments = typeParameters.map {
                         ConeTypeParameterTypeImpl(it.symbol.toLookupTag(), false).toFirResolvedTypeRef()