[SE-0160] Add an opt-in warning about deprecated @objc inference.

Introduce an opt-in warning (enabled by the frontend option
-warn-swift3-objc-inference) for each declaration for which @objc is
inferred based on Swift 3 rules that no longer apply after SE-0160.

lib/Frontend/CompilerInvocation.cpp

@@ -955,6 +955,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   }
 
   Opts.EnableAppExtensionRestrictions |= Args.hasArg(OPT_enable_app_extension);
+  Opts.WarnSwift3ObjCInference |= Args.hasArg(OPT_warn_swift3_objc_inference);
 
   llvm::Triple Target = Opts.Target;
   StringRef TargetArg;