[libc++] Clang-tidy enable modernize-use-nullptr. (#76659)

Clang-tidy 18 no longer has false positives with the spaceship operator.
Note that I'm quite sure there are more occurrences in our headers that
are not caught.

Author: mordante

libcxx/.clang-tidy

@@ -15,6 +15,7 @@ Checks: >
 
   modernize-loop-convert,
   modernize-redundant-void-arg,
+  modernize-use-nullptr,
   modernize-use-override,
 
   readability-duplicate-include,
@@ -68,7 +69,6 @@ CheckOptions:
 # modernize-use-default-member-init,
 # modernize-use-equals-default,
 # modernize-use-equals-delete,
-# modernize-use-nullptr,
 # portability-restrict-system-includes,
 # readability-function-cognitive-complexity,
 # readability-implicit-bool-conversion,

libcxx/include/__atomic/is_always_lock_free.h

@@ -20,7 +20,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _Tp>
 struct __libcpp_is_always_lock_free {
   // __atomic_always_lock_free is available in all Standard modes
-  static const bool __value = __atomic_always_lock_free(sizeof(_Tp), 0);
+  static const bool __value = __atomic_always_lock_free(sizeof(_Tp), nullptr);
 };
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/locale

@@ -3421,7 +3421,7 @@ wbuffer_convert<_Codecvt, _Elem, _Tr>::~wbuffer_convert() {
 template <class _Codecvt, class _Elem, class _Tr>
 typename wbuffer_convert<_Codecvt, _Elem, _Tr>::int_type wbuffer_convert<_Codecvt, _Elem, _Tr>::underflow() {
   _LIBCPP_SUPPRESS_DEPRECATED_POP
-  if (__cv_ == 0 || __bufptr_ == 0)
+  if (__cv_ == 0 || __bufptr_ == nullptr)
     return traits_type::eof();
   bool __initial = __read_mode();
   char_type __1buf;
@@ -3478,7 +3478,7 @@ template <class _Codecvt, class _Elem, class _Tr>
 typename wbuffer_convert<_Codecvt, _Elem, _Tr>::int_type
 wbuffer_convert<_Codecvt, _Elem, _Tr>::pbackfail(int_type __c) {
   _LIBCPP_SUPPRESS_DEPRECATED_POP
-  if (__cv_ != 0 && __bufptr_ != 0 && this->eback() < this->gptr()) {
+  if (__cv_ != 0 && __bufptr_ && this->eback() < this->gptr()) {
     if (traits_type::eq_int_type(__c, traits_type::eof())) {
       this->gbump(-1);
       return traits_type::not_eof(__c);
@@ -3496,7 +3496,7 @@ _LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Codecvt, class _Elem, class _Tr>
 typename wbuffer_convert<_Codecvt, _Elem, _Tr>::int_type wbuffer_convert<_Codecvt, _Elem, _Tr>::overflow(int_type __c) {
   _LIBCPP_SUPPRESS_DEPRECATED_POP
-  if (__cv_ == 0 || __bufptr_ == 0)
+  if (__cv_ == 0 || !__bufptr_)
     return traits_type::eof();
   __write_mode();
   char_type __1buf;
@@ -3588,7 +3588,7 @@ template <class _Codecvt, class _Elem, class _Tr>
 typename wbuffer_convert<_Codecvt, _Elem, _Tr>::pos_type
 wbuffer_convert<_Codecvt, _Elem, _Tr>::seekoff(off_type __off, ios_base::seekdir __way, ios_base::openmode __om) {
   int __width = __cv_->encoding();
-  if (__cv_ == 0 || __bufptr_ == 0 || (__width <= 0 && __off != 0) || sync())
+  if (__cv_ == 0 || !__bufptr_ || (__width <= 0 && __off != 0) || sync())
     return pos_type(off_type(-1));
   // __width > 0 || __off == 0, now check __way
   if (__way != ios_base::beg && __way != ios_base::cur && __way != ios_base::end)
@@ -3601,7 +3601,7 @@ wbuffer_convert<_Codecvt, _Elem, _Tr>::seekoff(off_type __off, ios_base::seekdir
 template <class _Codecvt, class _Elem, class _Tr>
 typename wbuffer_convert<_Codecvt, _Elem, _Tr>::pos_type
 wbuffer_convert<_Codecvt, _Elem, _Tr>::seekpos(pos_type __sp, ios_base::openmode __wch) {
-  if (__cv_ == 0 || __bufptr_ == 0 || sync())
+  if (__cv_ == 0 || !__bufptr_ || sync())
     return pos_type(off_type(-1));
   if (__bufptr_->pubseekpos(__sp, __wch) == pos_type(off_type(-1)))
     return pos_type(off_type(-1));
@@ -3611,7 +3611,7 @@ wbuffer_convert<_Codecvt, _Elem, _Tr>::seekpos(pos_type __sp, ios_base::openmode
 template <class _Codecvt, class _Elem, class _Tr>
 int wbuffer_convert<_Codecvt, _Elem, _Tr>::sync() {
   _LIBCPP_SUPPRESS_DEPRECATED_POP
-  if (__cv_ == 0 || __bufptr_ == 0)
+  if (__cv_ == 0 || !__bufptr_)
     return 0;
   if (__cm_ & ios_base::out) {
     if (this->pptr() != this->pbase())