fix complex typdesc iterators

tests/iter/titertypedesc.nim

@@ -0,0 +1,17 @@
+discard """
+  output: '''0
+(id: 0)
+@[]
+[0, 0, 0]'''
+"""
+
+iterator foo*(T: typedesc): T =
+  var x: T
+  yield x
+
+for a in foo(int): echo a
+for b in foo(tuple[id: int]): echo b
+for c in foo(seq[int]): echo c
+
+type Generic[T] = T
+for d in foo(Generic[array[0..2, int]]): echo d