Ensure that defining a function on an arg counts as a reference.

Otherwise you get a warning here saying that t is unused:

    (fn f [t x] (fn t.f []) x)

src/fennel-ls/compiler.fnl

@@ -80,7 +80,7 @@ later by fennel-ls.language to answer requests from the client."
             (table.insert target.referenced-by ast)))))
 
     (λ symbol-to-expression [ast scope ?reference?]
-      (if ?reference?
+      (when (or ?reference? (fennel.multi-sym? ast))
         (reference ast scope)))
 
     (λ define [?definition binding scope]

test/diagnostic-test.fnl

@@ -97,7 +97,7 @@
 
   (it "does not warn if a field is used"
     (let [self (create-client)
-          responses (self:open-file! filename "(fn [abc] (set abc.xyz 10))")]
+          responses (self:open-file! filename "(fn [a b] (set a.x 10) (fn b.f []))")]
       (assert (not (?. responses 1 :params :diagnostics 1)))))
 
   (it "warns when using the : special when a multisym would do"