Disable parsing of single quoted character literals, enabling under a flag.

I didn't want to rip this logic out wholesale.  There is a possibility
the character lexing can be reborn/revisited later, and
disabling it in the parser was easy.

Swift SVN r18102

lib/Frontend/CompilerInvocation.cpp

@@ -534,6 +534,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableExperimentalPatterns |= Args.hasArg(OPT_enable_experimental_patterns);
 
+  Opts.EnableCharacterLiterals |= Args.hasArg(OPT_enable_character_literals);
+
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
 
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);