emacs-lisp: Speed up semantic

Resolve syl20bnr#1907.

Emacs 25 sets semanticdb-find-default-throttle as a mode-local variable in
emacs-lisp-mode and includes the omniscient throttle which slows down indexing.
Let's override the mode-local variable so it doesn't include the omniscient
throttle.

See syl20bnr#7736 for additional context.

layers/+lang/emacs-lisp/packages.el

@@ -201,7 +201,10 @@
 (defun emacs-lisp/post-init-semantic ()
   (add-hook 'emacs-lisp-mode-hook 'semantic-mode)
   (with-eval-after-load 'semantic
-    (semantic-default-elisp-setup)))
+    (semantic-default-elisp-setup)
+    (setq-mode-local emacs-lisp-mode
+                     semanticdb-find-default-throttle
+                     '(file local project unloaded system))))
 
 (defun emacs-lisp/post-init-srefactor ()
   (add-hook 'emacs-lisp-mode-hook 'spacemacs/lazy-load-srefactor)