[libc++] Remove old workaround for buildit

[ldionne]

Summary

I'm not sure what the problem was at the time, however I don't think
this is necessary since buildit doesn't exist anymore.

Instead of the workaround, the correct thing to do is to leave out
the get_new_handler/set_new_handler definitions from libc++ when
we're getting them from libc++abi.

[ldionne]

libcxx/src/new.cpp

26 ↗

(On Diff #195202)

The reasoning here is that for Apple builds, we never define _LIBCPP_BUILDING_HAS_NO_ABI_LIBRARY (maybe we used to but we don't anymore). Hence, the conditional is equivalent to:

#if defined(__APPLE__) && 1

Furthermore, we always define _LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS for Apple builds, and nobody else I'm aware of does it, so this conditional is equivalent to

#if defined(_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS) && 1

Now we take the #else branch and that's the refactor.

The only thing this could break is if someone builds libc++ with _LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS but their ABI library doesn't provide get_new_handler/set_new_handler. However, I'm not aware of any such user (and frankly if they exist then they're probably following these reviews).

[EricWF]

I think this is fine.

[mclow.lists]

This looks ok to me.

libcxx/trunk/src/new.cpp

 #   endif
 #elif defined(LIBCXX_BUILDING_LIBCXXABI)
 #   include <cxxabi.h>
 #elif defined(LIBCXXRT)
 #   include <cxxabi.h>
 #   include "support/runtime/new_handler_fallback.ipp"
 #elif defined(__GLIBCXX__)
     // nothing to do
-#else
-#   if defined(__APPLE__) && !defined(_LIBCPP_BUILDING_HAS_NO_ABI_LIBRARY)
-#       include <cxxabi.h> // FIXME: remove this once buildit is gone.
-#   else
-#       include "support/runtime/new_handler_fallback.ipp"
-#   endif
-#endif
+#elif !defined(_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS)
+#   include "support/runtime/new_handler_fallback.ipp"
+#endif
 
 namespace std
 {
 
 #ifndef __GLIBCXX__
 const nothrow_t nothrow{};
 #endif