Don't use std::errc.

As noted on Errc.h:

// * std::errc is just marked with is_error_condition_enum. This means that
//   common patters like AnErrorCode == errc::no_such_file_or_directory take
//   4 virtual calls instead of two comparisons.

And on some libstdc++ those virtual functions conclude that

------------------------
int main() {
  std::error_code foo = std::make_error_code(std::errc::no_such_file_or_directory);
  return foo == std::errc::no_such_file_or_directory;
}
-------------------------

should exit with 0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239683 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/ErrorOrTest.cpp

@@ -67,8 +67,8 @@ TEST(ErrorOr, Covariant) {
 }
 
 TEST(ErrorOr, Comparison) {
-  ErrorOr<int> x(std::errc::no_such_file_or_directory);
-  EXPECT_EQ(x, std::errc::no_such_file_or_directory);
+  ErrorOr<int> x(errc::no_such_file_or_directory);
+  EXPECT_EQ(x, errc::no_such_file_or_directory);
 }
 
 // ErrorOr<int*> x(nullptr);