Change default error_code ctor to a 'named ctor' so it's more self-documenting.

Unify default construction of error_code uses on this idiom so that users don't
feel compelled to make static globals for naming convenience. (unfortunately I
couldn't make the original ctor private as some APIs don't return their result,
instead using an out parameter (that makes sense to default construct) - which
is a bit of a pity. I did, however, find/fix some cases of unnecessary default
construction of error_code before I hit the unfixable cases)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150197 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/DataStream.cpp

@@ -67,7 +67,7 @@ public:
     if (Filename == "-") {
       Fd = 0;
       sys::Program::ChangeStdinToBinary();
-      return error_code();
+      return error_code::success();
     }
   
     int OpenFlags = O_RDONLY;
@@ -77,7 +77,7 @@ public:
     Fd = ::open(Filename.c_str(), OpenFlags);
     if (Fd == -1)
       return error_code(errno, posix_category());
-    return error_code();
+    return error_code::success();
   }
 };