Improve the 'modernize-use-default-member-init'

Improve the 'modernize-use-default-member-init'

We want to deal with non-default constructors that just happen to
contain constant initializers. There was already a negative test case,
it is now a positive one. We find and refactor this case:

struct PositiveNotDefaultInt {
  PositiveNotDefaultInt(int) : i(7) {}
  int i;
};