Improvements to emacs packages for llvm and tablegen mode.

* Both files have valid package headers and footers (you can verify
with M-x checkdoc).
* Fixed style warnings generated by checkdoc.
* Fixed a byte-compiler warning in llvm-mode.el.
* Ensure that the modes are autoloaded, so users do not need to
(require 'llvm-mode) to use them.

Patch by Wilfred Hughes.

llvm-svn: 225356