More missing header inclusions from llvm_unreachable migration.

llvm-svn: 140369
diff --git a/clang/lib/Frontend/TextDiagnosticBuffer.cpp b/clang/lib/Frontend/TextDiagnosticBuffer.cpp
index c03ac54..f540fca 100644
--- a/clang/lib/Frontend/TextDiagnosticBuffer.cpp
+++ b/clang/lib/Frontend/TextDiagnosticBuffer.cpp
@@ -13,6 +13,7 @@
 
 #include "clang/Frontend/TextDiagnosticBuffer.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/Support/ErrorHandling.h"
 using namespace clang;
 
 /// HandleDiagnostic - Store the errors, warnings, and notes that are