Auto merge of #8163 - pmnoxx:piotr-improve-unwrap-or-default, r=Manishearth

Improve `unwrap_or_else_default` when handling `unwrap_or_else(XXX::new)`

changelog:  change `unwrap_or_else_default` to work with std constructors like `Vec::new`, `HashSet::new`, `HashMap::new`.

Notes:
- Code to handle detecting those constructors is already there. I moved it out to `is_default_equivalent_call`