Merge pull request #8557 from ian-twilightcoder/cxx-unreachable

[cherry-pick swift/release/6.0][Headers] Don't declare unreachable() from stddef.h in C++

Author: fredriss

clang/lib/Headers/__stddef_unreachable.h

@@ -7,6 +7,8 @@
  *===-----------------------------------------------------------------------===
  */
 
+#ifndef __cplusplus
+
 /*
  * When -fbuiltin-headers-in-system-modules is set this is a non-modular header
  * and needs to behave as if it was textual.
@@ -15,3 +17,5 @@
     (__has_feature(modules) && !__building_module(_Builtin_stddef))
 #define unreachable() __builtin_unreachable()
 #endif
+
+#endif