[libc++] Handle 0 size case for testing support operator new (#93834)

The return of malloc is implementation defined when the requested size
is 0. On platforms (such as AIX) that return a null pointer for 0 size,
operator new will throw a bad_alloc exception. operator new should
return a non null pointer for 0 size instead.

Author: jakeegan

libcxx/test/std/algorithms/alg.sorting/alg.merge/inplace_merge.pass.cpp

@@ -14,8 +14,6 @@
 //   void
 //   inplace_merge(Iter first, Iter middle, Iter last);
 
-// XFAIL: LIBCXX-AIX-FIXME
-
 #include <algorithm>
 #include <cassert>
 #include <random>

libcxx/test/support/count_new.h

@@ -385,6 +385,8 @@ MemCounter &globalMemCounter = *getGlobalMemCounter();
 // operator new(size_t[, nothrow_t]) and operator delete(size_t[, nothrow_t])
 void* operator new(std::size_t s) TEST_THROW_SPEC(std::bad_alloc) {
   getGlobalMemCounter()->newCalled(s);
+  if (s == 0)
+    ++s;
   void* p = std::malloc(s);
   if (p == nullptr)
     detail::throw_bad_alloc_helper();
@@ -417,6 +419,8 @@ void operator delete(void* p, std::nothrow_t const&) TEST_NOEXCEPT {
 // operator new[](size_t[, nothrow_t]) and operator delete[](size_t[, nothrow_t])
 void* operator new[](std::size_t s) TEST_THROW_SPEC(std::bad_alloc) {
   getGlobalMemCounter()->newArrayCalled(s);
+  if (s == 0)
+    s++;
   void* p = std::malloc(s);
   if (p == nullptr)
     detail::throw_bad_alloc_helper();