[clang] [clang][Headers] add endian.h (PR #186032)

[Hubert Tong via cfe-commits]

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@@ -0,0 +1,62 @@
+//===-- Definition of macros from endian.h --------------------------------===//
+//
+// 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 __CLANG_ENDIAN_H
+#define __CLANG_ENDIAN_H
+
+// If the system has an endian.h, let's use that instead.
+#if __STDC_HOSTED__ && __has_include_next(<endian.h>)
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hubert-reinterpretcast wrote:

Informational only: This is consistent with `stdckdint.h`, which uses this pattern and is not a standard header required for a conforming freestanding implementation of C.

https://github.com/llvm/llvm-project/pull/186032