properly serialize/parse Nothing type

compiler/testData/writeSignature/Nothing.kt

@@ -0,0 +1,6 @@
+fun nothing(): Nothing = throw Exception()
+
+// method: namespace::nothing
+// jvm signature:     ()V
+// generic signature: null
+// kotlin signature:  ()Ljet/Nothing;

compiler/testData/writeSignature/NothingQ.kt

@@ -0,0 +1,6 @@
+fun nothingq(): Nothing? = null
+
+// method: namespace::nothingq
+// jvm signature:     ()Ljava/lang/Object;
+// generic signature: null
+// kotlin signature:  ()?Ljet/Nothing;