[libcxx] Apply namespace-scope hidden visibility to libc++

[ldionne]

Summary

This is technically not a functional change because all symbols that
should be exported from the shared library are already marked with
explicit visibility annotations. However, applying this attribute
at the namespace scope will allow us to drop many visibility
annotations in the future, which is a nice simplification.

This commit also solves the problem of different symbols being exported
depending on the build mode in use (see PR38138). With this change, stray
symbols are not exported from the dylib due to implicit instantiations,
and as a result we export the same symbols in Debug and Release modes.

[EricWF]

Would it make sense to remove the -fvisibility-inlines-hidden from the build at the same time? Currently doing so causes 37 previously hidden functions to be exported.

[ldionne]

In D54810#1305831, @EricWF wrote:

Would it make sense to remove the -fvisibility-inlines-hidden from the build at the same time?

Yes, it would.

Currently doing so causes 37 previously hidden functions to be exported.

Argh. I'll look into that tomorrow. Note that this also checks out with previous experiments I've done where removing hidden visibility attributes on some functions after making the whole namespace hidden result in similar problems.

[ldionne]

Abandoning since we should use #pragma clang attribute push with a visibility attribute instead.

libcxx/include/__config

 #else
 #  define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI
 #endif
 
 // Just so we can migrate to the new macros gradually.
 #define _LIBCPP_INLINE_VISIBILITY _LIBCPP_HIDE_FROM_ABI
 
 // Inline namespaces are available in Clang/GCC/MSVC regardless of C++ dialect.
-#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std { inline namespace _LIBCPP_ABI_NAMESPACE {
+#define _LIBCPP_BEGIN_NAMESPACE_STD namespace std _LIBCPP_HIDDEN { inline namespace _LIBCPP_ABI_NAMESPACE {
 #define _LIBCPP_END_NAMESPACE_STD  } }
 #define _VSTD std::_LIBCPP_ABI_NAMESPACE
 _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 17
 #define _LIBCPP_BEGIN_NAMESPACE_FILESYSTEM \
   _LIBCPP_BEGIN_NAMESPACE_STD inline namespace __fs { namespace filesystem {
 #else