[Frontend] Rename EnableObjCAttrRequiresObjCModule -> EnableObjCAttrRequiresFoundation and enable it by default.

This is enabled by default because SILGen can crash when @objc is used without importing Foundation, but
it gets disabled when compiling the Swift stdlib.

Addresses rdar://17931250.
Related test case on the SourceKit side.

Swift SVN r21319

lib/Frontend/CompilerInvocation.cpp

@@ -609,7 +609,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   
   if (auto A = Args.getLastArg(OPT_enable_objc_attr_requires_foundation_module,
                                OPT_disable_objc_attr_requires_foundation_module)) {
-    Opts.EnableObjCAttrRequiresObjCModule
+    Opts.EnableObjCAttrRequiresFoundation
       = A->getOption().matches(OPT_enable_objc_attr_requires_foundation_module);
   }