Sema: Finish staging back in availability checking in synthesized functions

Remove the suppression of deprecation and potential unavailability diagnostics in
synthesized functions. We still suppress some explicit unavailability diagnostics -- those
in synthesized functions in synthesized functions that are lexically contained in
declarations that are themselves annotated as unavailable. For these cases, the right
solution <rdar://problem/20491640> is to not synthesize the bodies of these functions in
the first place.

rdar://problem/20024980

Swift SVN r27203

lib/Frontend/CompilerInvocation.cpp

@@ -608,9 +608,6 @@ 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);