Add initializers to postfix-expr code completions

Previously, the only way to get initializers was completing after the
name of the type:
    Foo#^complete_here^#
    Foo(#^or_here^#

And now it will also work in unadorned expressions:
    #^a_top_level_completion^#
    bar(a, #^walked_into_a_bar^#

Unfortunately, not all our clients handle this well yet, so it's
protected by a language option.
    -code-complete-inits-in-postfix-expr

Swift SVN r27275

lib/Frontend/CompilerInvocation.cpp

@@ -623,7 +623,10 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.EnableAccessControl
       = A->getOption().matches(OPT_enable_access_control);
   }
-  
+
+  Opts.CodeCompleteInitsInPostfixExpr |=
+      Args.hasArg(OPT_code_complete_inits_in_postfix_expr);
+
   if (auto A = Args.getLastArg(OPT_enable_target_os_checking,
                                OPT_disable_target_os_checking)) {
     Opts.EnableTargetOSChecking