Fix missing includes for llvm_unreachable


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140368 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Lex/PPMacroExpansion.cpp b/lib/Lex/PPMacroExpansion.cpp
index ce4c1ed..cfe9374 100644
--- a/lib/Lex/PPMacroExpansion.cpp
+++ b/lib/Lex/PPMacroExpansion.cpp
@@ -25,6 +25,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Support/ErrorHandling.h"
 #include <cstdio>
 #include <ctime>
 using namespace clang;