[libc++] Add visibility annotations to the std namespace with GCC (#133233)

This allows us to remove the need for `_LIBCPP_TEMPLATE_VIS` and fixes a
bunch of missing annotations for RTTI when used across dylib boundaries.
`_LIBCPP_TEMPLATE_VIS` itself will be removed in a separate patch, since
it touches a lot of code.

This patch is a no-op for Clang. Only GCC is affected.

Author: philnik777

libcxx/docs/ReleaseNotes/21.rst

@@ -77,7 +77,10 @@ LLVM 22
 ABI Affecting Changes
 ---------------------
 
-- TODO
+- When using GCC, the ``std`` namespace is now annotated with ``[[gnu::visibility("default")]]``. This may cause more
+  symbols to be exported from shared libraries when building with ``-fvisibility=hidden``. This also fixes RTTI
+  comparison between shared libraries, since all RTTI has the correct visibility now. There is no behaviour change on
+  Clang.
 
 
 Build System Changes

libcxx/include/__config

@@ -386,7 +386,7 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
 #    define _LIBCPP_TEMPLATE_VIS
 #    define _LIBCPP_TEMPLATE_DATA_VIS
-#    define _LIBCPP_TYPE_VISIBILITY_DEFAULT
+#    define _LIBCPP_NAMESPACE_VISIBILITY
 
 #  else
 
@@ -414,17 +414,16 @@ typedef __char32_t char32_t;
 #      define _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
 #    endif
 
-// GCC doesn't support the type_visibility attribute, so we have to keep the visibility attribute on templates
-#    if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS) && !__has_attribute(__type_visibility__)
-#      define _LIBCPP_TEMPLATE_VIS __attribute__((__visibility__("default")))
-#    else
-#      define _LIBCPP_TEMPLATE_VIS
-#    endif
+// This is kept to avoid a huge library-wide diff in the first step.
+// TODO: Remove this in a follow-up patch
+#    define _LIBCPP_TEMPLATE_VIS
 
 #    if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS) && __has_attribute(__type_visibility__)
-#      define _LIBCPP_TYPE_VISIBILITY_DEFAULT __attribute__((__type_visibility__("default")))
+#      define _LIBCPP_NAMESPACE_VISIBILITY __attribute__((__type_visibility__("default")))
+#    elif !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
+#      define _LIBCPP_NAMESPACE_VISIBILITY __attribute__((__visibility__("default")))
 #    else
-#      define _LIBCPP_TYPE_VISIBILITY_DEFAULT
+#      define _LIBCPP_NAMESPACE_VISIBILITY
 #    endif
 
 #  endif // defined(_LIBCPP_OBJECT_FORMAT_COFF)
@@ -583,7 +582,7 @@ typedef __char32_t char32_t;
 // If it's not clear whether using the unversioned namespace is the correct thing to do, it's not. The versioned
 // namespace (_LIBCPP_BEGIN_NAMESPACE_STD) should almost always be used.
 #  define _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD                                                                      \
-    _LIBCPP_PUSH_EXTENSION_DIAGNOSTICS namespace _LIBCPP_TYPE_VISIBILITY_DEFAULT std {
+    _LIBCPP_PUSH_EXTENSION_DIAGNOSTICS namespace _LIBCPP_NAMESPACE_VISIBILITY std {
 
 #  define _LIBCPP_END_UNVERSIONED_NAMESPACE_STD } _LIBCPP_POP_EXTENSION_DIAGNOSTICS