Set treesit-primary-parser for c and elixir ts mode

Set treesit-primary-parser for c and elixir ts mode

For buffers with multiple parsers, it's important to set this variable
so font-lock invalidation works smoothly.

* lisp/progmodes/c-ts-mode.el (c-ts-mode): Set treesit-primary-parser.
* lisp/progmodes/elixir-ts-mode.el (elixir-ts-mode): Set
treesit-primary-parser.

(cherry picked from commit 3cad7cc8dc823b8b1dd66d7777a01778e1a4defe)