Allow recursive closure iterators (#19939)

tests/iter/titer_issues.nim

@@ -29,6 +29,7 @@ end
 9018
 @[1, 2]
 @[1, 2, 3]
+1
 '''
 """
 
@@ -274,3 +275,15 @@ iterator cc() {.closure.} =
             break
 
 var a2 = cc
+
+# bug #16876
+block:
+  iterator a(num: int): int {.closure.} =
+      if num == 1:
+          yield num
+      else:
+          for i in a(num - 1):
+              yield i
+
+  for i in a(5):
+    echo i