[libc++] Introduce _LIBCPP_EXTERN_VIS to fix __libcpp_debug_function link errors

[EricWF]

Summary

On Windows tests that use _LIBCPP_ASSERT fail to link because the assertion handler function isn't correctly exported from the libc++ dylib. This patch fixes the dll import/export issues by introducing a new visibility macro _LIBCPP_EXTERN_VIS for use on external variables.

[smeenai]

This will need to be exported on other platforms too when hidden visibility happens :)

Is it okay for these to be exported unconditionally, or should their exporting be determined by _LIBCPP_DEBUG? This LGTM if it's the former.

[EricWF]

In D28728#646489, @smeenai wrote:

This will need to be exported on other platforms too when hidden visibility happens :)

Are you sure?I don't think "extern" variables are subject to visibility rules in GCC or Clang. At least I haven't
been able to generate a test case or even different assembly.

Is it okay for these to be exported unconditionally, or should their exporting be determined by _LIBCPP_DEBUG? This LGTM if it's the former.

Unconditionally. The declaration should already be #ifdef'ed out when libc++'s debug mode is not enabled.

I'm going to commit this as-is, and will address visibility issues on other platforms if needed.

docs/DesignDocs/VisibilityMacros.rst

 
 **_LIBCPP_HIDDEN**
   Mark a symbol as hidden so it will not be exported from shared libraries.
 
 **_LIBCPP_FUNC_VIS**
   Mark a symbol as being exported by the libc++ library. This attribute must
   be applied to the declaration of all functions exported by the libc++ dylib.
 
+**_LIBCPP_EXTERN_VIS**
+  Mark a symbol as being exported by the libc++ library. This attribute may
+  only be applied to objects defined in the libc++ library. On Windows this
+  macro applies `dllimport`/`dllexport` to the symbol. On all other platforms
+  this macro has no effect.
+
 **_LIBCPP_OVERRIDABLE_FUNC_VIS**
   Mark a symbol as being exported by the libc++ library, but allow it to be
   overridden locally. On non-Windows, this is equivalent to `_LIBCPP_FUNC_VIS`.
   This macro is applied to all `operator new` and `operator delete` overloads.
 
   **Windows Behavior**: Any symbol marked `dllimport` cannot be overridden
   locally, since `dllimport` indicates the symbol should be bound to a separate
   DLL. All `operator new` and `operator delete` overloads are required to be

include/__config

 # define _LIBCPP_DLL_VIS __declspec(dllimport)
 # define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
 # define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 # define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #endif
 
 #define _LIBCPP_TYPE_VIS            _LIBCPP_DLL_VIS
 #define _LIBCPP_FUNC_VIS            _LIBCPP_DLL_VIS
+#define _LIBCPP_EXTERN_VIS          _LIBCPP_DLL_VIS
 #define _LIBCPP_EXCEPTION_ABI       _LIBCPP_DLL_VIS
 #define _LIBCPP_HIDDEN
 #define _LIBCPP_TEMPLATE_VIS
 #define _LIBCPP_FUNC_VIS_ONLY
 #define _LIBCPP_ENUM_VIS
 
 #if defined(_LIBCPP_COMPILER_MSVC)
 # define _LIBCPP_INLINE_VISIBILITY __forceinline

 #ifndef _LIBCPP_TEMPLATE_VIS
 # define _LIBCPP_TEMPLATE_VIS _LIBCPP_TYPE_VIS
 #endif
 
 #ifndef _LIBCPP_FUNC_VIS_ONLY
 # define _LIBCPP_FUNC_VIS_ONLY _LIBCPP_FUNC_VIS
 #endif
 
+#ifndef _LIBCPP_EXTERN_VIS
+# define _LIBCPP_EXTERN_VIS
+#endif
+
 #ifndef _LIBCPP_OVERRIDABLE_FUNC_VIS
 # define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_FUNC_VIS
 #endif
 
 #ifndef _LIBCPP_EXCEPTION_ABI
 #if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 #define _LIBCPP_EXCEPTION_ABI __attribute__ ((__visibility__("default")))
 #else

include/__debug

struct _LIBCPP_TEMPLATE_VIS __libcpp_debug_info {
   const char* __msg_;
 };
 
 /// __libcpp_debug_function_type - The type of the assertion failure handler.
 typedef void(*__libcpp_debug_function_type)(__libcpp_debug_info const&);
 
 /// __libcpp_debug_function - The handler function called when a _LIBCPP_ASSERT
 ///    fails.
-extern __libcpp_debug_function_type __libcpp_debug_function;
+extern _LIBCPP_EXTERN_VIS __libcpp_debug_function_type __libcpp_debug_function;
 
 /// __libcpp_abort_debug_function - A debug handler that aborts when called.
 _LIBCPP_NORETURN _LIBCPP_FUNC_VIS
 void __libcpp_abort_debug_function(__libcpp_debug_info const&);
 
 /// __libcpp_throw_debug_function - A debug handler that throws
 ///   an instance of __libcpp_debug_exception when called.
  _LIBCPP_NORETURN _LIBCPP_FUNC_VIS