[libc] [llvm] [libc] Move libc_errno.h to libc/src/__support and make LIBC_ERRNO_MODE_SYSTEM to be header-only. (PR #143187)

[via llvm-commits]

================
@@ -0,0 +1,20 @@
+//===-- Common defines for sharing LLVM libc with LLVM projects -*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_LIBC_COMMON_H
+#define LLVM_LIBC_SHARED_LIBC_COMMON_H
+
+// Use system errno.
+#undef LIBC_ERRNO_MODE
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_SYSTEM
----------------
lntue wrote:

I guess we can enforce that with clang-tidy check and bot?

https://github.com/llvm/llvm-project/pull/143187