Update -enable-ossa-modules to be a module interface ignoreable option

This lets the flag be emitted in the swiftinterface file of the stdlib
(or other modules with -enable-ossa-modules specified),
so that whenever stdlib is recompiled, -enable-ossa-modules will be on.

Author: meg-gupta

include/swift/Option/FrontendOptions.td

@@ -1159,9 +1159,11 @@ def disable_ast_verifier : Flag<["-"], "disable-ast-verifier">,
            "disabled. NOTE: Can not be used if enable-ast-verifier is used "
            "as well">;
 
-def enable_ossa_modules : Flag<["-"], "enable-ossa-modules">,
-  HelpText<"Always serialize SIL in ossa form. If this flag is not passed in, "
-           "when optimizing ownership will be lowered before serializing SIL">;
+let Flags = [FrontendOption, NoDriverOption, HelpHidden, ModuleInterfaceOptionIgnorable] in {
+  def enable_ossa_modules : Flag<["-"], "enable-ossa-modules">,
+    HelpText<"Always serialize SIL in ossa form. If this flag is not passed in, "
+            "when optimizing ownership will be lowered before serializing SIL">;
+}
 
 def enable_sil_opaque_values : Flag<["-"], "enable-sil-opaque-values">,
   HelpText<"Enable SIL Opaque Values">;