[libc][NFC] Use the custom operator new from strndup.

libc/src/string/CMakeLists.txt

add_entrypoint_object(
   SRCS
     strndup.cpp
   HDRS
     strndup.h
   DEPENDS
     .memory_utils.memcpy_implementation
     .string_utils
     libc.include.stdlib
+    libc.src.__support.CPP.new
 )
 
 add_entrypoint_object(
   strnlen
   SRCS
     strnlen.cpp
   HDRS
     strnlen.h

libc/src/string/strndup.cpp

 //===-- Implementation of strndup -----------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "src/string/strndup.h"
 #include "src/string/memory_utils/memcpy_implementations.h"
 #include "src/string/string_utils.h"
 
+#include "src/__support/CPP/new.h"
 #include "src/__support/common.h"
 
 #include <stddef.h>
-#include <stdlib.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(char *, strndup, (const char *src, size_t size)) {
   if (src == nullptr)
     return nullptr;
   size_t len = internal::string_length(src);
   if (len > size)
     len = size;
-  char *dest = reinterpret_cast<char *>(::malloc(len + 1));
-  if (dest == nullptr)
+  AllocChecker ac;
+  char *dest = new (ac) char[len];
+  if (!ac)
     return nullptr;
   inline_memcpy(dest, src, len + 1);
   dest[len] = '\0';
   return dest;
 }
 
 } // namespace __llvm_libc