[libc++][format] Renames __null_sentinel.

[Mordante]

Summary

While the FreeBSD CI was enabled in D128084 it was discovered libc++
uses the name of a system macro on FreeBSD. This renames the macro to
fix the issue.

[philnik]

libcxx/test/libcxx/nasty_macros.compile.pass.cpp

63

[Mordante]

Fixes copy-paste error spotted by philnik.

[Mordante]

libcxx/test/libcxx/nasty_macros.compile.pass.cpp

63

Thanks!

[emaste]

This fixes the majority of the failures I observed while trying to bring up FreeBSD CI

[ldionne]

libcxx/include/__format/formatter_output.h

376

I think this was a typo? If so, can you please fix in a NFC?

[Mordante]

libcxx/include/__format/formatter_output.h

376

nul instead of null was intentionally. It looks for the NUL character, so actually the older name was "wrong".

libcxx/include/__format/formatter_output.h

_LIBCPP_HIDE_FROM_ABI auto __write_string(
 
   int __size = __formatter::__truncate(__str, __specs.__precision_);
 
   return __write(__str.begin(), __str.end(), _VSTD::move(__out_it), __specs, __size);
 }
 
 #  if _LIBCPP_STD_VER > 20
 
-struct __null_sentinel {};
+struct __nul_terminator {};
 
 template <class _CharT>
-_LIBCPP_HIDE_FROM_ABI bool operator==(const _CharT* __cstr, __null_sentinel) {
+_LIBCPP_HIDE_FROM_ABI bool operator==(const _CharT* __cstr, __nul_terminator) {
   return *__cstr == _CharT('\0');
 }
 
 template <class _CharT>
 _LIBCPP_HIDE_FROM_ABI void
 __write_escaped_code_unit(basic_string<_CharT>& __str, char32_t __value, const _CharT* __prefix) {
   back_insert_iterator __out_it{__str};
-  std::ranges::copy(__prefix, __null_sentinel{}, __out_it);
+  std::ranges::copy(__prefix, __nul_terminator{}, __out_it);
 
   char __buffer[8];
   to_chars_result __r = std::to_chars(std::begin(__buffer), std::end(__buffer), __value, 16);
   _LIBCPP_ASSERT(__r.ec == errc(0), "Internal buffer too small");
   std::ranges::copy(std::begin(__buffer), __r.ptr, __out_it);
 
   __str += _CharT('}');
 }

libcxx/test/libcxx/nasty_macros.compile.pass.cpp

 #endif
 
 // FreeBSD's <sys/types.h> uses _M
 //
 #ifdef __FreeBSD__
 # undef _M
 #endif
 
+// Test that libc++ doesn't use names that collide with FreeBSD system macros.
+#ifndef __FreeBSD__
+#  define __null_sentinel NASTY_MACRO
+#endif
+
 // tchar.h defines these macros on Windows
 #ifndef _WIN32
 # define _UI   NASTY_MACRO
 # define _PUC  NASTY_MACRO
 # define _CPUC NASTY_MACRO
 # define _PC   NASTY_MACRO
 # define _CRPC NASTY_MACRO
 # define _CPC  NASTY_MACRO