Add support for driver option -mno-ms-bitfields.

This option is used to cancel -mms-bitfields on the command line.

rdar://problem/15898553

llvm-svn: 252912
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index e25321b..e821de5 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -3713,7 +3713,8 @@
   if (KernelOrKext && getToolChain().getTriple().isOSDarwin())
     CmdArgs.push_back("-fforbid-guard-variables");
 
-  if (Args.hasArg(options::OPT_mms_bitfields)) {
+  if (Args.hasFlag(options::OPT_mms_bitfields, options::OPT_mno_ms_bitfields,
+                   false)) {
     CmdArgs.push_back("-mms-bitfields");
   }