Don't use std::errc

Summary:
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.

Reviewers: thakis, rnk, jfb

Reviewed By: thakis

Subscribers: lebedev.ri, dexonsmith, xbolva00, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66143

llvm-svn: 368739