Make static initializer classes for inheriting multifile class parts annotated as SYNTHETIC_CLASS.

compiler/backend/src/org/jetbrains/kotlin/codegen/MultifileClassCodegen.kt

@@ -78,7 +78,7 @@ class MultifileClassCodegen(
             state.fileClassesProvider.getFileClassInfo(this).fileClassFqName
 
     private val shouldGeneratePartHierarchy =
-            state.inheritMultifileParts // TODO support incremental compilation
+            state.inheritMultifileParts
 
     private val partInternalNamesSorted = run {
         val partInternalNamesSet = hashSetOf<String>()

compiler/backend/src/org/jetbrains/kotlin/codegen/MultifileClassPartCodegen.kt

@@ -115,6 +115,8 @@ class MultifileClassPartCodegen(
                     visitMaxs(1, 0)
                     visitEnd()
                 }
+
+                writeSyntheticClassMetadata(this)
             }
         }
     }