[libc++] Handle _LIBCPP_HAS_NO_{THREADS,LOCALIZATION} consistently with other carve-outs (#98319)

Previously, we would issue an #error when using a header that requires
threading support or localization support in a configuration where that
is disabled. This is unlike what we do for all the other carve outs like
no-filesystem, no-wide-characters or no-random-device. Instead of
issuing an #error, we normally just remove the problematic parts of the
header.

This patch makes the handling of no-localization and no-threads
consistent with the other carve-outs. I dislike the fact that users
won't get an explicit error message when trying to use e.g. ios in a
build that doesn't support localization, but I think it is better to
handle things consistently. Note that besides the consistency argument,
the #error approach doesn't really work anyways since it would break
down if we moved towards assuming the C locale only in the
no-localization mode.

Author: ldionne

libcxx/docs/ReleaseNotes/19.rst

@@ -134,6 +134,12 @@ Deprecations and Removals
   `std-allocator-const <https://clang.llvm.org/extra/clang-tidy/checks/portability/std-allocator-const.html>`
   enabled.
 
+- When configuring libc++ with localization or threads disabled, the library no longer emits an error when
+  trying to ``#include <locale>`` and other such headers. Instead, those headers have no content. This is
+  consistent with the behavior for all other libc++ carve-outs like filesystem, wide characters, a source
+  of randomness, and others. Users that were checking whether including a header would fail (e.g. via a script
+  or CMake's ``try_compile`` will experience a change in behavior).
+
 
 Upcoming Deprecations and Removals
 ----------------------------------

libcxx/include/barrier

@@ -47,38 +47,36 @@ namespace std
 
 #include <__config>
 
-#ifdef _LIBCPP_HAS_NO_THREADS
-#  error "<barrier> is not supported since libc++ has been configured without support for threads."
-#endif
-
-#include <__assert>
-#include <__atomic/atomic_base.h>
-#include <__atomic/memory_order.h>
-#include <__memory/unique_ptr.h>
-#include <__thread/poll_with_backoff.h>
-#include <__thread/timed_backoff_policy.h>
-#include <__utility/move.h>
-#include <cstddef>
-#include <cstdint>
-#include <limits>
-#include <version>
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
+#if !defined(_LIBCPP_HAS_NO_THREADS)
+
+#  include <__assert>
+#  include <__atomic/atomic_base.h>
+#  include <__atomic/memory_order.h>
+#  include <__memory/unique_ptr.h>
+#  include <__thread/poll_with_backoff.h>
+#  include <__thread/timed_backoff_policy.h>
+#  include <__utility/move.h>
+#  include <cstddef>
+#  include <cstdint>
+#  include <limits>
+#  include <version>
+
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
 
 _LIBCPP_PUSH_MACROS
-#include <__undef_macros>
+#  include <__undef_macros>
 
-#if _LIBCPP_STD_VER >= 14
+#  if _LIBCPP_STD_VER >= 14
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct __empty_completion {
   inline _LIBCPP_HIDE_FROM_ABI void operator()() noexcept {}
 };
 
-#  ifndef _LIBCPP_HAS_NO_TREE_BARRIER
+#    ifndef _LIBCPP_HAS_NO_TREE_BARRIER
 
 /*
 
@@ -152,7 +150,7 @@ public:
   }
 };
 
-#  else
+#    else
 
 /*
 
@@ -253,7 +251,7 @@ public:
   }
 };
 
-#  endif // !_LIBCPP_HAS_NO_TREE_BARRIER
+#    endif // !_LIBCPP_HAS_NO_TREE_BARRIER
 
 template <class _CompletionF = __empty_completion>
 class _LIBCPP_DEPRECATED_ATOMIC_SYNC barrier {
@@ -265,7 +263,7 @@ public:
   static _LIBCPP_HIDE_FROM_ABI constexpr ptrdiff_t max() noexcept { return __barrier_base<_CompletionF>::max(); }
 
   _LIBCPP_AVAILABILITY_SYNC
-      _LIBCPP_HIDE_FROM_ABI explicit barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
+  _LIBCPP_HIDE_FROM_ABI explicit barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
       : __b_(__count, std::move(__completion)) {
     _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
         __count >= 0,
@@ -292,10 +290,12 @@ public:
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // _LIBCPP_STD_VER >= 14
+#  endif // _LIBCPP_STD_VER >= 14
 
 _LIBCPP_POP_MACROS
 
+#endif // !defined(_LIBCPP_HAS_NO_THREADS)
+
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <atomic>
 #  include <concepts>