[libc++] Always build the dylib with hidden visibility (#131313)

The comment seems to be wrong by now. The only difference this makes for
GCC is that there are fewer private symbols exported from the dylib,
which can't ever be accessed by user code anyways.

Author: philnik777

libcxx/CMakeLists.txt

@@ -522,13 +522,7 @@ function(cxx_add_basic_build_flags target)
   # the dylib when get ODR used by another function.
   target_add_compile_flags_if_supported(${target} PRIVATE -fvisibility-inlines-hidden)
 
-  # Our visibility annotations are not quite right for non-Clang compilers,
-  # so we end up not exporting all the symbols we should. In the future, we
-  # can improve the situation by providing an explicit list of exported
-  # symbols on all compilers.
-  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-    target_add_compile_flags_if_supported(${target} PRIVATE -fvisibility=hidden)
-  endif()
+  target_add_compile_flags_if_supported(${target} PRIVATE -fvisibility=hidden)
 
   # Build with -fsized-deallocation, which is default in recent versions of Clang.
   # TODO(LLVM 21): This can be dropped once we only support Clang >= 19.