[FIR] Introduce different synthetic origins for FIR declarations

This is needed to be able to precisely distinguish different synthetic
  declarations (like fields for delegates and functions for data classes)
  in fir2ir

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/session/NativeForwardDeclarationsSymbolProvider.kt

@@ -87,7 +87,7 @@ class NativeForwardDeclarationsSymbolProvider(
 
         buildRegularClass {
             moduleData = forwardDeclarationsModuleData
-            origin = FirDeclarationOrigin.Synthetic
+            origin = FirDeclarationOrigin.Synthetic.ForwardDeclaration
             check(!classId.isNestedClass) { "Expected top-level class when building forward declaration, got $classId" }
             name = classId.shortClassName
             status = FirResolvedDeclarationStatusImpl(