More missing header inclusions from llvm_unreachable migration.

llvm-svn: 140369
diff --git a/clang/lib/Driver/Action.cpp b/clang/lib/Driver/Action.cpp
index 65faa4e..a7feed1 100644
--- a/clang/lib/Driver/Action.cpp
+++ b/clang/lib/Driver/Action.cpp
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Driver/Action.h"
+#include "llvm/Support/ErrorHandling.h"
 
 #include <cassert>
 using namespace clang::driver;