[libc++] Do not forward-declare syncstream outside experimental

We only define the classes in <syncstream> when experimental
library features are enabled, but we would forward-declare
them in <iosfwd> even when they are disabled. This led to
confusing error messages about being unable to instantiate
an undefined template.

Author: ldionne

libcxx/include/iosfwd

@@ -143,7 +143,7 @@ typedef fpos<mbstate_t> u8streampos;
 typedef fpos<mbstate_t> u16streampos;
 typedef fpos<mbstate_t> u32streampos;
 
-#if _LIBCPP_STD_VER >= 20
+#if _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM)
 
 template <class _CharT, class _Traits = char_traits<_CharT>, class _Allocator = allocator<_CharT>>
 class basic_syncbuf;
@@ -161,7 +161,7 @@ using osyncstream = basic_osyncstream<char>;
 using wosyncstream = basic_osyncstream<wchar_t>;
 #  endif
 
-#endif // _LIBCPP_STD_VER >=20
+#endif // _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_EXPERIMENTAL_SYNCSTREAM)
 
 // Include other forward declarations here
 template <class _Tp, class _Alloc = allocator<_Tp> >