Documentation cleanup: escape #undef in Doxygen comments.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158625 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/Basic/MacroBuilder.h b/include/clang/Basic/MacroBuilder.h
index 8b552cf..cb58376 100644
--- a/include/clang/Basic/MacroBuilder.h
+++ b/include/clang/Basic/MacroBuilder.h
@@ -29,8 +29,8 @@
     Out << "#define " << Name << ' ' << Value << '\n';
   }
 
-  /// Append a #undef line for Name.  Name should be of the form XXX
-  /// and we emit "#undef XXX".
+  /// Append a \#undef line for Name.  Name should be of the form XXX
+  /// and we emit "\#undef XXX".
   void undefineMacro(const Twine &Name) {
     Out << "#undef " << Name << '\n';
   }