Revert "Don't save the sign ID on loclist items. This approach won't work."

This reverts commit f25a543260.

test/test_linting_updates_loclist.vader

@@ -13,6 +13,7 @@ Before:
   \   'type': 'W',
   \   'col': 10,
   \   'text': 'Infix operators must be spaced. [Warning/space-infix-ops]',
+  \   'sign_id': 1000001,
   \ },
   \ {
   \   'lnum': 2,
@@ -23,6 +24,7 @@ Before:
   \   'type': 'E',
   \   'col': 10,
   \   'text': 'Missing semicolon. [Error/semi]',
+  \   'sign_id': 1000002,
   \ }
   \]