[emacs] Add regexps for defuns

Previously `M-x mark-defun` and friends wouldn't work properly, and would
highlight something at the top of the buffer. This adds a regexp for top-level
functions so the defun functions should work as expected, and also adds a
regexp for extracting their name so which-function-mode should work now too.

Reviewed By: goldstein.w.n

Differential Revision: https://reviews.llvm.org/D158196