Fix some regular expressions in llvm-mode.el.

In some cases it was using "\" unnecessarily.  In another case it
needed an additional "\" to properly indicate a numbered sub-match.

Make comment-start buffer-local in llvm-mode.el

llvm-mode was setting comment-start globally.  However, it is better
to only set it locally in the current buffer.

Don't use purecopy in llvm-mode.el

There's no reason to use purecopy in llvm-mode.el.
purecopy is only needed for files that are dumped in emacs.

Add a version header to llvm-mode.el

Adding a version header to llvm-mode.el allows it to be installed by
the Emacs package manager.  There are not many requirements on the
version number; however it is useful to users to bump it when
something significant changes.  Here I've chosen just to start at 1.0.

Patch by Tom Tromey!

llvm-svn: 323705