Only perform the null-initialization of an aggregate result of a message

Only perform the null-initialization of an aggregate result of a message
send if the receiver is null.  Normally it's not worthwhile to check this,
but avoiding the null-initialization is nice, and this also avoids nasty
problems where the null-initialization is visible within the call because
we use an aliased result buffer.  rdar://problem/9402992

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131366 91177308-0d34-0410-b5e6-96231b3b80d8