Include system_error directly.

llvm-svn: 210802
diff --git a/clang/lib/Lex/PTHLexer.cpp b/clang/lib/Lex/PTHLexer.cpp
index 42629d4..844d66a 100644
--- a/clang/lib/Lex/PTHLexer.cpp
+++ b/clang/lib/Lex/PTHLexer.cpp
@@ -25,8 +25,8 @@
 #include "llvm/Support/EndianStream.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/OnDiskHashTable.h"
-#include "llvm/Support/system_error.h"
 #include <memory>
+#include <system_error>
 using namespace clang;
 
 #define DISK_TOKEN_SIZE (1+1+2+4+4)