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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@368739 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Lex/HeaderSearch.cpp

@@ -30,6 +30,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Capacity.h"
+#include "llvm/Support/Errc.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
@@ -315,9 +316,9 @@ const FileEntry *HeaderSearch::getFileAndSuggestModule(
     // For rare, surprising errors (e.g. "out of file handles"), diag the EC
     // message.
     std::error_code EC = File.getError();
-    if (EC != std::errc::no_such_file_or_directory &&
-        EC != std::errc::invalid_argument && EC != std::errc::is_a_directory &&
-        EC != std::errc::not_a_directory) {
+    if (EC != llvm::errc::no_such_file_or_directory &&
+        EC != llvm::errc::invalid_argument &&
+        EC != llvm::errc::is_a_directory && EC != llvm::errc::not_a_directory) {
       Diags.Report(IncludeLoc, diag::err_cannot_open_file)
           << FileName << EC.message();
     }