[Headers] remove define of _GNU_SOURCE when including Darwin's unwind.h

[mhjacobson]

Summary

Darwin's unwind.h has never surrounded declarations with _GNU_SOURCE guards.
The _GNU_SOURCE stuff removed here was introduced by a09e62a042, which expanded
the #include_next branch to non-Darwin platforms, including those that use
https://github.com/libunwind/libunwind (which *does* use _GNU_SOURCE guards).
That change was partially reverted by 032d422d2e, which left the _GNU_SOURCE
stuff as an "open question".

Therefore: close the question; remove the _GNU_SOURCE define.

clang/lib/Headers/unwind.h

 /*===---- unwind.h - Stack unwinding ----------------------------------------===
  *
  * 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
  *
  *===-----------------------------------------------------------------------===
  */
 
 /* See "Data Definitions for libgcc_s" in the Linux Standard Base.*/
 
 #ifndef __CLANG_UNWIND_H
 #define __CLANG_UNWIND_H
 
 #if defined(__APPLE__) && __has_include_next(<unwind.h>)
 /* Darwin (from 11.x on) provide an unwind.h. If that's available,
- * use it. libunwind wraps some of its definitions in #ifdef _GNU_SOURCE,
- * so define that around the include.*/
-# ifndef _GNU_SOURCE
-#  define _SHOULD_UNDEFINE_GNU_SOURCE
-#  define _GNU_SOURCE
-# endif
+ * use it. */
+
 // libunwind's unwind.h reflects the current visibility.  However, Mozilla
 // builds with -fvisibility=hidden and relies on gcc's unwind.h to reset the
 // visibility to default and export its contents.  gcc also allows users to
 // override its override by #defining HIDE_EXPORTS (but note, this only obeys
 // the user's -fvisibility setting; it doesn't hide any exports on its own).  We
 // imitate gcc's header here:
 # ifdef HIDE_EXPORTS
 #  include_next <unwind.h>
 # else
 #  pragma GCC visibility push(default)
 #  include_next <unwind.h>
 #  pragma GCC visibility pop
 # endif
-# ifdef _SHOULD_UNDEFINE_GNU_SOURCE
-#  undef _GNU_SOURCE
-#  undef _SHOULD_UNDEFINE_GNU_SOURCE
-# endif
 #else
 
 #include <stdint.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif