Don't use 'using std::error_code' in include/llvm.

This should make sure that most new uses use the std prefix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210835 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/LockFileManager.cpp

@@ -22,6 +22,7 @@
 #include <unistd.h>
 #endif
 using namespace llvm;
+using std::error_code;
 
 /// \brief Attempt to read the lock file with the given name, if it exists.
 ///