Emit @objc classes using namespaced names under a new flag -enable-objc-mangling.

Centralize the logic for figuring out what name to use for a class or
protocol in the Objective-C runtime. When the flag is enabled (it's
still disabled by default), use mangled names for all Swift-defined
classes, including those that are @objc. Note that the naming is
determined in the AST, because we're also going to use this logic when
printing an Objective-C header for Clang's consumption. The mangled
names will always start with _Tt, so they're easy to recognize and
demangle in various tools or, eventually, in the Objective-C runtime.

The new test (test/IRGen/objc_mangling.sil) is the only test of this
behavior at the moment. The other test changes are due to the
centralized logic tweaking the names of internal constants (_DATA_*,
_CATEGORY_*, etc.).

This is the majority of <rdar://problem/15506580>.



Swift SVN r15588

lib/Frontend/CompilerInvocation.cpp

@@ -508,6 +508,12 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.EnableExperimentalPatterns = true;
   }
 
+  if (const Arg *A = Args.getLastArg(OPT_enable_objc_mangling, 
+                                     OPT_disable_objc_mangling)) {
+    Opts.MangleObjCClassProtocolNames 
+      = A->getOption().matches(OPT_enable_objc_mangling);
+  }
+
   if (Args.hasArg(OPT_debug_constraints)) {
     Opts.DebugConstraintSolver = true;
   }