Add @cpp.AllowLegacyDeprecatedTerseWritesRef to allow using cpp.Ref w…

…ith deprecated_terse_writes cpp2 option

Summary: We will ban such usages without this annotation.

Reviewed By: thedavekwon

Differential Revision: D68802575

fbshipit-source-id: 16a60085e3951f0f3ce64e46040d210b9ef93cc9

thrift/annotation/cpp.thrift

@@ -403,3 +403,13 @@ struct AllowLegacyNonOptionalRef {}
  */
 @scope.Field
 struct DeprecatedTerseWrite {}
+
+/**
+ * Allows the field to be annotated @cpp.Ref (or cpp[2].ref[_type]) even if it
+ * is deprecated_terse_writes field.
+ *
+ * This annotation is provided for a limited time, to exempt pre-existing fields
+ * while rolling out a stricter enforcement of the condition above.
+ */
+@scope.Field
+struct AllowLegacyDeprecatedTerseWritesRef {}