Use aliases for more constructors and destructors.

With this patch we produce alias for cases like

template<typename T>
struct foobar {
  foobar() {
  }
};
template struct foobar<void>;

It is safe to use aliases to weak symbols, as long and the alias itself is also
weak.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192300 91177308-0d34-0410-b5e6-96231b3b80d8