[SE-0160] Allow one to enable/disable Swift 3 @objc inference rules.

Introduce flags `-enable-swift3-objc-inference` and
`-disable-swift3-objc-inference` to enable/disable the Swift 3 `@objc`
inference rules. Under `-swift-version 3`, default to the former;
under `-swift-version 4`, default to the latter. For testing purposes,
one can provide either flag in eiher language mode.

lib/Frontend/CompilerInvocation.cpp

@@ -957,6 +957,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableAppExtensionRestrictions |= Args.hasArg(OPT_enable_app_extension);
   Opts.WarnSwift3ObjCInference |= Args.hasArg(OPT_warn_swift3_objc_inference);
 
+  Opts.EnableSwift3ObjCInference =
+    Args.hasFlag(OPT_enable_swift3_objc_inference,
+                 OPT_disable_swift3_objc_inference,
+                 Opts.isSwiftVersion3());
+
   llvm::Triple Target = Opts.Target;
   StringRef TargetArg;
   if (const Arg *A = Args.getLastArg(OPT_target)) {