Prohibit inner classes in objects

Using inner classes in objects makes little sense since objects have single static instance

compiler/testData/diagnostics/tests/inner/illegalModifier.kt

@@ -27,12 +27,12 @@ trait K {
 }
 
 object N {
-    inner class O
+    <!INNER_CLASS_IN_OBJECT!>inner<!> class O
 }
 
 class P {
     class object {
-        inner class Q
+        <!INNER_CLASS_IN_OBJECT!>inner<!> class Q
     }
 }
 

compiler/testData/diagnostics/tests/inner/illegalModifier.txt

@@ -136,7 +136,7 @@ internal object N {
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 
-    internal final inner class O {
+    internal final class O {
         public constructor O()
         public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
@@ -156,7 +156,7 @@ internal final class P {
         public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
         public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 
-        internal final inner class Q {
+        internal final class Q {
             public constructor Q()
             public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
             public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int