Don't inherit dll attributes to deleted methods (PR19988)

We would previously end up with an error when instantiating the
following template:

  template <typename> struct __declspec(dllimport) S {
    void foo() = delete;
  };
  S<int> s;

error: attribute 'dllimport' cannot be applied to a deleted function
Test commit, wraps some lines to fit in 80 columns.