[libc++] Enable aligned allocation based on feature test macro, irrespective of standard

Summary:
The current code enables aligned allocation functions when compiling in C++17
and later. This is a problem because aligned allocation functions might not
be supported on the target platform, which leads to an error at link time.

Since r338934, Clang knows not to define __cpp_aligned_new when it's not
available on the target platform -- this commit takes advantage of that to
only use aligned allocation functions when they are available.

Reviewers: vsapsai, EricWF

Subscribers: christof, dexonsmith, cfe-commits, EricWF, mclow.lists

Differential Revision: https://reviews.llvm.org/D50344

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@339431 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/memory/aligned_allocation_macro.pass.cpp

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+// XFAIL: with_system_cxx_lib=macosx10.12
+// XFAIL: with_system_cxx_lib=macosx10.11
+// XFAIL: with_system_cxx_lib=macosx10.10
+// XFAIL: with_system_cxx_lib=macosx10.9
+// XFAIL: with_system_cxx_lib=macosx10.8
+// XFAIL: with_system_cxx_lib=macosx10.7
+
+#include <new>
+
+
+#ifdef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
+#   error "libc++ should have aligned allocation in C++17 and up when targeting a platform that supports it"
+#endif
+
+int main() { }