[FIR] add support for implementation by delgation

This commit handles "subclass: super-interface by delegate-expression".

During Psi2Fir, for each delegate, we add to the subclass a synthetic
field (which has type super-interface), and an assignment of the
delegate-expression to the synthetic field in the primary constructor,
so that the delegate-expression can be resolved and transformed along
the way.

During Fir2Ir, we look up delegatable members from the super-interface
and generate corresponding functions/properties for the subclass.

TODO: support for generic delegatable members and generic
super-interface.

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirDeclarationUtil.kt

@@ -9,8 +9,8 @@ import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.fir.declarations.builder.FirRegularClassBuilder
 import org.jetbrains.kotlin.fir.declarations.builder.FirTypeParameterBuilder
-import org.jetbrains.kotlin.fir.declarations.impl.FirRegularClassImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirFileImpl
+import org.jetbrains.kotlin.fir.declarations.impl.FirRegularClassImpl
 import org.jetbrains.kotlin.fir.symbols.impl.FirAnonymousObjectSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
@@ -119,4 +119,7 @@ var FirProperty.isFromVararg: Boolean? by FirDeclarationDataRegistry.data(IsFrom
 
 fun FirAnnotatedDeclaration.hasAnnotation(classId: ClassId): Boolean {
     return annotations.any { it.annotationTypeRef.coneTypeSafe<ConeClassLikeType>()?.classId == classId }
-}
+}
+
+inline val FirDeclaration.isSynthetic: Boolean
+    get() = origin == FirDeclarationOrigin.Synthetic