debuginfo: Test cases for [generic][default][static] methods and functions:

* closure-in-generic-function
* generic-functions-nested
* generic-method-on-generic-struct
* generic-trait-generic-static-default-method
* method-on-generic-struct
* self-in-generic-default-method
* trait-generic-static-default-method

Also, fixed an 'unused variable' warning in debuginfo.rs