Sema: Add temporary flag to stage in availability checking in implicit functions

This flag enables checking of availability (deprecation, explicit unavailability,
and potential unavailability) in synthesized functions. The flag will go away once this
checking is fully staged in.

Swift SVN r26624

lib/Frontend/CompilerInvocation.cpp

@@ -606,6 +606,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   
   Opts.EnableExperimentalUnavailableAsOptional |=
       Args.hasArg(OPT_enable_experimental_unavailable_as_optional);
+
+  Opts.EnableAvailabilityCheckingInImplicitFunctions |=
+      Args.hasArg(OPT_enable_availability_checking_in_implicit_functions);
   
   Opts.EnableCFunctionPointers |=
       Args.hasArg(OPT_enable_c_function_pointers);