Add driver flags -ftrigraphs, -fno-trigraphs.

-trigraphs is now an alias for -ftrigraphs.  -fno-trigraphs makes it possible
to explicitly disable trigraphs, which couldn't be done before.

  clang -std=c++11 -fno-trigraphs

now builds without GNU extensions, but with trigraphs disabled.  Previously,
trigraphs were only disabled in GNU modes or with -std=c++1z.

Make the new -f flags the cc1 interface too.  This requires changing -trigraphs
to -ftrigraphs in a few cc1 tests.

Related to PR21974.

llvm-svn: 224790

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1440,8 +1440,8 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   // is specified, or -std is set to a conforming mode.
   // Trigraphs are disabled by default in c++1z onwards.
   Opts.Trigraphs = !Opts.GNUMode && !Opts.MSVCCompat && !Opts.CPlusPlus1z;
-  if (Args.hasArg(OPT_trigraphs))
-    Opts.Trigraphs = 1;
+  Opts.Trigraphs =
+      Args.hasFlag(OPT_ftrigraphs, OPT_fno_trigraphs, Opts.Trigraphs);
 
   Opts.DollarIdents = Args.hasFlag(OPT_fdollars_in_identifiers,
                                    OPT_fno_dollars_in_identifiers,