generate-proxy-class.php should generate the same file regardless of a missing leading slash

Problem/Motivation

php core/scripts/generate-proxy-class.php takes two arguments, a class name and a directory.

When providing a leading slash for the class name, like \Drupal\foo\FooInterface, some of the generated docblocks will have two leading slashes, like \\Drupal\foo\FooInterface.
If you do not provide a leading slash, the @file block will be missing the leading slash.

Proposed resolution

Be consistent, and handle both cases the same

Remaining tasks

User interface changes

API changes

Data model changes