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

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

llvm-svn: 210835
diff --git a/llvm/lib/Support/Unix/Program.inc b/llvm/lib/Support/Unix/Program.inc
index f4c6cdf..d850664 100644
--- a/llvm/lib/Support/Unix/Program.inc
+++ b/llvm/lib/Support/Unix/Program.inc
@@ -48,6 +48,8 @@
 #endif
 
 namespace llvm {
+using std::error_code;
+
 using namespace sys;
 
 ProcessInfo::ProcessInfo() : Pid(0), ReturnCode(0) {}