Start importing Objective-C initializers and factory methods using failable initializers.

In this mode, use nullability information on the result type of the
initializer or factory method to determine failability of the
initializer. This is behind the flag
-enable-objc-failable-initializers until we have the SILGen support in
place.



Swift SVN r21341

lib/Frontend/CompilerInvocation.cpp

@@ -664,6 +664,8 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts, ArgList &Args,
 
   Opts.InferImplicitProperties |=
     Args.hasArg(OPT_enable_objc_implicit_properties);
+  Opts.UseFailableInitializers |=
+    Args.hasArg(OPT_enable_objc_failable_initializers);
 
   Opts.DumpClangDiagnostics |= Args.hasArg(OPT_dump_clang_diagnostics);