Add _tags file for OSS build

Summary: This was originally added in {D1762313} but subsequently
reverted due to lack of OCaml 3.12 support. Now that we've dropped
support for 3.12, we can use it!

Reviewed By: @jwatzman

Differential Revision: D2098483

hphp/hack/src/_tags

@@ -0,0 +1,14 @@
+<**/*.ml*>: ocaml, warn_A, warn(-4-6-29-35-44-48), warn_error_A
+<*.ml*>: warn(-27)
+<client/*.ml*>: warn(-27)
+<deps/*.ml*>: warn(-27)
+<dfind/*.ml*>: warn(-27)
+<format/*.ml*>: warn(-27)
+<heap/*.ml*>: warn(-27-34)
+<naming/*.ml*>: warn(-27-45)
+<parsing/*.ml*>: warn(-27)
+<procs/*.ml*>: warn(-27)
+<server/*.ml*>: warn(-27)
+<third-party/avl/*.ml*>: warn(-27)
+<third-party/core/*.ml*>: warn(-32)
+<utils/*.ml*>: warn(-3-27)