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

include/llvm/Support/system_error.h

@@ -738,6 +738,10 @@ class error_code {
 public:
   error_code() : _val_(0), _cat_(&system_category()) {}
 
+  static error_code success() {
+    return error_code();
+  }
+
   error_code(int _val, const error_category& _cat)
     : _val_(_val), _cat_(&_cat) {}