[libc++] Add option to disable new/delete overloads when libc++abi provides them.

Summary:
Currently both libc++ and libc++abi provide definitions for operator new/delete. However I believe this is incorrect and that one or the other should offer them.

This patch adds the CMake option `-DLIBCXX_ENABLE_NEW_DELETE_DEFINITIONS` which defaults no `ON` unless `-DLIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS=ON` is specified.



Reviewers: mclow.lists, mehdi_amini, dexonsmith, danalbert, smeenai, mgorny, rmaprath

Reviewed By: mehdi_amini

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30516

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@296802 91177308-0d34-0410-b5e6-96231b3b80d8

src/new.cpp

@@ -53,7 +53,8 @@ __throw_bad_alloc()
 
 }  // std
 
-#if !defined(__GLIBCXX__) && !defined(_LIBCPP_ABI_MICROSOFT)
+#if !defined(__GLIBCXX__) && !defined(_LIBCPP_ABI_MICROSOFT) && \
+    !defined(_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS)
 
 // Implement all new and delete operators as weak definitions
 // in this shared library, so that they can be overridden by programs
@@ -298,4 +299,4 @@ operator delete[] (void* ptr, size_t, std::align_val_t alignment) _NOEXCEPT
 }
 
 #endif // !_LIBCPP_HAS_NO_ALIGNED_ALLOCATION
-#endif // !__GLIBCXX__ && !_LIBCPP_ABI_MICROSOFT
+#endif // !__GLIBCXX__ && !_LIBCPP_ABI_MICROSOFT && !_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS