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/PPExpressions.cpp b/lib/Lex/PPExpressions.cpp
index 120b2ca..66a12ec 100644
--- a/lib/Lex/PPExpressions.cpp
+++ b/lib/Lex/PPExpressions.cpp
@@ -23,6 +23,7 @@
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Lex/LexDiagnostic.h"
 #include "llvm/ADT/APSInt.h"
+#include "llvm/Support/ErrorHandling.h"
 using namespace clang;
 
 namespace {