[Headers] Don't declare unreachable() from stddef.h in C++ (llvm#86748)

Even if __need_unreachable is set, stddef.h should not declare
unreachable() in C++ because it conflicts with the declaration in
\<utility>.

(cherry picked from commit df69a30)

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