[Driver] Add -fno-digraphs

Summary: Add a flag `-fno-digraphs` to disable digraphs in the lexer, similar to `-fno-operator-names` which disables alternative names for C++ operators.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: rsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D48266

llvm-svn: 337232
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index f57f23a..d62429d 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -2173,6 +2173,8 @@
   Opts.GNUKeywords = Args.hasFlag(OPT_fgnu_keywords, OPT_fno_gnu_keywords,
                                   Opts.GNUKeywords);
 
+  Opts.Digraphs = Args.hasFlag(OPT_fdigraphs, OPT_fno_digraphs, Opts.Digraphs);
+
   if (Args.hasArg(OPT_fno_operator_names))
     Opts.CXXOperatorNames = 0;