Introduce UnsafeVariance annotation

core/builtins/src/kotlin/Annotations.kt

@@ -119,6 +119,14 @@ public annotation class HiddenDeclaration
 @MustBeDocumented
 private annotation class external
 
+/**
+ * Suppresses errors about variance conflict
+ */
+@Target(TYPE)
+@Retention(SOURCE)
+@MustBeDocumented
+public annotation class UnsafeVariance
+
 /**
  * Specifies that the corresponding type should be ignored during type inference.
  */