Fix operator= on OwningAtomPtr to call destructor when needed.

If the LHS of 'a = b' already had an atom in it then we wouldn't
call the destructor.  This happens when we use something like
std::remove_if which is done in the CompactUnwindPass.  Should fix
the leaks on the mach-o/unwind-info-simple-x86_64.yaml test case.

Lang and I are going to take a look at removing OwningAtomPtr in
favour of a std::unique_ptr but just trying to get the bots green
so we have a good baseline first.

llvm-svn: 264097