patch 8.2.2584: Vim9: type error for assigning "any" to a list

Problem:    Vim9: type error for assigning the result of list concatenation to
            a list.
Solution:   Do not consider concatenation result in a constant. (closes #7942)

src/testdir/test_vim9_expr.vim

@@ -1370,6 +1370,9 @@ def Test_expr5_list_add()
     dany[i] = i
   endfor
   assert_equal({a: 'a', 12: 12}, dany)
+
+  # result of glob() is "any", runtime type check
+  var sl: list<string> = glob('*.txt', false, true) + ['']
 enddef
 
 " test multiply, divide, modulo