[libcxxabi] Fix the test case committed in r303175.

Free the __cxa_exception object allocated with __cxa_allocate_exception.

This is an attempt to fix this asan bot:

http://lab.llvm.org:8011/builders/libcxx-libcxxabi-x86_64-linux-ubuntu-asan/builds/560

llvm-svn: 303194

libcxxabi/test/exception_object_alignment.pass.cpp

@@ -28,6 +28,7 @@ int main() {
   auto i = reinterpret_cast<uintptr_t>(p);
   auto a = std::alignment_of<S>::value;
   assert(i % a == 0);
+  __cxxabiv1::__cxa_free_exception(p);
 #endif
   return 0;
 }