[libc++] Two internal headers are still using `NULL`

[frederick-vs-ja]

llvm-project/libcxx/include/__support/xlocale/__nop_locale_mgmt.h

Lines 18 to 24 in a75565a

	inline _LIBCPP_HIDE_FROM_ABI locale_t duplocale(locale_t) { return NULL; }
	inline _LIBCPP_HIDE_FROM_ABI void freelocale(locale_t) {}
	inline _LIBCPP_HIDE_FROM_ABI locale_t newlocale(int, const char*, locale_t) { return NULL; }
	inline _LIBCPP_HIDE_FROM_ABI locale_t uselocale(locale_t) { return NULL; }

llvm-project/libcxx/include/__locale_dir/locale_base_api/ibm.h

Lines 82 to 106 in a75565a

	inline _LIBCPP_HIDE_FROM_ABI
	_LIBCPP_ATTRIBUTE_FORMAT(__printf__, 2, 0) int vasprintf(char** strp, const char* fmt, va_list ap) {
	const size_t buff_size = 256;
	if ((*strp = (char*)malloc(buff_size)) == NULL) {
	return -1;
	}
	va_list ap_copy;
	// va_copy may not be provided by the C library in C++03 mode.
	#if defined(_LIBCPP_CXX03_LANG) && __has_builtin(__builtin_va_copy)
	__builtin_va_copy(ap_copy, ap);
	#else
	va_copy(ap_copy, ap);
	#endif
	int str_size = vsnprintf(*strp, buff_size, fmt, ap_copy);
	va_end(ap_copy);
	if ((size_t)str_size >= buff_size) {
	if ((*strp = (char*)realloc(*strp, str_size + 1)) == NULL) {
	return -1;
	}
	str_size = vsnprintf(*strp, str_size + 1, fmt, ap);
	}
	return str_size;
	}

Given libc++'s <__config> supports nullptr in ancient C++03 mode

llvm-project/libcxx/include/__config

Line 324 in a75565a

# define nullptr __nullptr

Should we just replace these uses of NULL with nullptr?