patch 8.2.3276: Vim9: exists() can only be evaluated at runtime

Problem:    Vim9: exists() can only be evaluated at runtime.
Solution:   Evaluate at compile time for option name literals. (closes #8437)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index ff0d33a..6db930e 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -49,7 +49,6 @@
 static void f_eval(typval_T *argvars, typval_T *rettv);
 static void f_eventhandler(typval_T *argvars, typval_T *rettv);
 static void f_execute(typval_T *argvars, typval_T *rettv);
-static void f_exists(typval_T *argvars, typval_T *rettv);
 static void f_expand(typval_T *argvars, typval_T *rettv);
 static void f_expandcmd(typval_T *argvars, typval_T *rettv);
 static void f_feedkeys(typval_T *argvars, typval_T *rettv);
@@ -3521,7 +3520,7 @@
 /*
  * "exists()" function
  */
-    static void
+    void
 f_exists(typval_T *argvars, typval_T *rettv)
 {
     char_u	*p;