[libc++][NFC] Remove _LIBCPP_DLL_VIS

This macro is only ever used inside the definiton for the various
visibility macros on windows. There, it's defined in multiple places
with different expansions, which makes it more confusing than helpful
when trying to figure out what macro expands to what.

Author: philnik777

libcxx/include/__config

@@ -363,25 +363,22 @@ typedef __char32_t char32_t;
 #    endif
 
 #    if defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS) || (defined(__MINGW32__) && !defined(_LIBCPP_BUILDING_LIBRARY))
-#      define _LIBCPP_DLL_VIS
 #      define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 #      define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #      define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #      define _LIBCPP_EXPORTED_FROM_ABI
 #    elif defined(_LIBCPP_BUILDING_LIBRARY)
-#      define _LIBCPP_DLL_VIS __declspec(dllexport)
 #      if defined(__MINGW32__)
-#        define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
+#        define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __declspec(dllexport)
 #        define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #      else
 #        define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
-#        define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DLL_VIS
+#        define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS __declspec(dllexport)
 #      endif
-#      define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_DLL_VIS
+#      define _LIBCPP_OVERRIDABLE_FUNC_VIS __declspec(dllexport)
 #      define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllexport)
 #    else
-#      define _LIBCPP_DLL_VIS __declspec(dllimport)
-#      define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
+#      define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __declspec(dllimport)
 #      define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #      define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #      define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllimport)