[libc++] Give extern templates default visibility on gcc

Contrary to the current visibility macro documentation, it appears that
gcc does handle visibility attribute on extern templates correctly, e.g.
https://godbolt.org/g/EejuV7. We need this so that extern template
instantiations of classes not marked _LIBCPP_TEMPLATE_VIS (e.g.
__vector_base_common) are correctly exported with gcc when building with
hidden visibility.

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D35388

Author: smeenai

libcxx/docs/DesignDocs/VisibilityMacros.rst

@@ -131,12 +131,6 @@ Visibility Macros
   specified on the primary template and to export the member functions produced
   by the explicit instantiation in the dylib.
 
-  **GCC Behavior**: GCC ignores visibility attributes applied the type in
-  extern template declarations and applying an attribute results in a warning.
-  However since `_LIBCPP_TEMPLATE_VIS` is the same as
-  `__attribute__((visibility("default"))` the visibility is already correct.
-  The macro has an empty definition with GCC.
-
   **Windows Behavior**: `extern template` and `dllexport` are fundamentally
   incompatible *on a class template* on Windows; the former suppresses
   instantiation, while the latter forces it. Specifying both on the same

libcxx/include/__config

@@ -718,7 +718,7 @@ typedef __char32_t char32_t;
 #endif
 
 #ifndef _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
-#  if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS) && __has_attribute(__type_visibility__)
+#  if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 #    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __attribute__ ((__visibility__("default")))
 #  else
 #    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS