Add --enable-throw-without-try option to suppress warning about missing 'try'.

To be used by the debugger so expression parsing doesn't require 'try'
everywhere. <rdar://problem/20356764>

Swift SVN r27949

lib/Frontend/CompilerInvocation.cpp

@@ -667,6 +667,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.EnableDollarIdentifiers = true;
   Opts.Playground |= Args.hasArg(OPT_playground);
 
+  Opts.EnableThrowWithoutTry |= Args.hasArg(OPT_enable_throw_without_try);
+
   if (auto A = Args.getLastArg(OPT_enable_objc_attr_requires_foundation_module,
                                OPT_disable_objc_attr_requires_foundation_module)) {
     Opts.EnableObjCAttrRequiresFoundation