Forbid private functions with no body and private properties in traits which are abstract by default

compiler/testData/diagnostics/tests/modifiers/privateInInterface.kt

@@ -0,0 +1,12 @@
+interface My {
+    <!PRIVATE_PROPERTY_IN_TRAIT!>private<!> val x: Int
+    <!INCOMPATIBLE_MODIFIERS!>private<!> <!ABSTRACT_MODIFIER_IN_TRAIT, INCOMPATIBLE_MODIFIERS!>abstract<!> val xx: Int
+    private val xxx: Int
+        get() = 0
+    <!FINAL_PROPERTY_IN_TRAIT, DEPRECATED_MODIFIER_CONTAINING_DECLARATION!>final<!> val y: Int
+    <!DEPRECATED_MODIFIER_CONTAINING_DECLARATION!>final<!> val yy: Int
+        get() = 1
+    <!PRIVATE_FUNCTION_WITH_NO_BODY!>private<!> fun foo(): Int
+    // ok
+    private fun bar() = 42
+}

compiler/testData/diagnostics/tests/modifiers/privateInInterface.txt

@@ -0,0 +1,14 @@
+package
+
+public interface My {
+    private abstract val x: kotlin.Int
+    private abstract val xx: kotlin.Int
+    private final val xxx: kotlin.Int
+    public final val y: kotlin.Int
+    public final val yy: kotlin.Int
+    private final fun bar(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    private abstract fun foo(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}