[patch] Use aliases for more constructors and destructors

[Rafael Espíndola]

I noticed that gcc uses aliases in, for example:

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

clang is currently way too conservative. It only uses aliases when the
symbols are strong. It is safe to use aliases to weak symbols, as long
and the alias itself is also weak.

The attached patch implements that.

Cheers,
Rafael
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