Add completion for ProvidesPackage, ProvidesClass, and ProvidesFile.

* latex.el (TeX-arg-file-name): New function.
(TeX-arg-file-name-sans-extension): Ditto.
(TeX-arg-version): Ditto.
(LaTeX-common-initialization): Add completion for
`ProvidesPackage', `ProvidesClass', and `ProvidesFile'.

* doc/auctex.texi (Adding Macros): Document `TeX-arg-version',
`TeX-arg-file-name', and `TeX-arg-file-name-sans-extension'.