[libc++] Use the no_destroy attribute to avoid destroying debug DB statics

[ldionne]

Summary

Otherwise, we can run into problems when the program has static variables
that need to use the debug database during their deinitialization, if
the debug DB has already been deinitialized.

[ldionne]

I am concerned that we're going to start flagging leaks because we don't destroy those -- do you share the concern? I mean I can also just push and see what our various sanitizers say.

[EricWF]

It may start being reported as a leak by tooling, in the same way static T* t = new T would, which is the other idiomatic way to do this.

If an LLVM tool reports this as a leak, I think we should fix the tool.

include/__config

 #ifndef _LIBCPP_CONSTEXPR_IF_NODEBUG
 #if defined(_LIBCPP_DEBUG) || defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
 #define _LIBCPP_CONSTEXPR_IF_NODEBUG
 #else
 #define _LIBCPP_CONSTEXPR_IF_NODEBUG constexpr
 #endif
 #endif
 
+#if __has_attribute(no_destroy)
+#  define _LIBCPP_NO_DESTROY __attribute__((__no_destroy__))
+#else
+#  define _LIBCPP_NO_DESTROY
+#endif
+
 #ifndef _LIBCPP_HAS_NO_ASAN
 _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
   const void *, const void *, const void *, const void *);
 #endif
 
 // Try to find out if RTTI is disabled.
 // g++ and cl.exe have RTTI on by default and define a macro when it is.
 // g++ only defines the macro in 4.3.2 and onwards.

src/debug.cpp

bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
   __libcpp_debug_function = __func;
   return true;
 }
 
 _LIBCPP_FUNC_VIS
 __libcpp_db*
 __get_db()
 {
-    static __libcpp_db db;
+    static _LIBCPP_NO_DESTROY __libcpp_db db;
     return &db;
 }
 
 _LIBCPP_FUNC_VIS
 const __libcpp_db*
 __get_const_db()
 {
     return __get_db();
 }
 
 namespace
 {
 
 #ifndef _LIBCPP_HAS_NO_THREADS
 typedef mutex mutex_type;
 typedef lock_guard<mutex_type> WLock;
 typedef lock_guard<mutex_type> RLock;
 
 mutex_type&
 mut()
 {
-    static mutex_type m;
+    static _LIBCPP_NO_DESTROY mutex_type m;
     return m;
 }
 #endif // !_LIBCPP_HAS_NO_THREADS
 
 }  // unnamed namespace
 
 __i_node::~__i_node()
 {