fixes #13583; enforce void for nkWhileStmt (#20947)

* fixes #13583; enfore void for nkWhileStmt

* one more case

tests/discard/tdiscardable.nim

@@ -65,3 +65,37 @@ proc main2() =
 
 main1()
 main2()
+
+block: # bug #13583
+  block:
+    proc hello(): int {.discardable.} = 12
+
+    iterator test(): int {.closure.} =
+      while true:
+        hello()
+
+    let t = test
+
+  block:
+    proc hello(): int {.discardable.} = 12
+
+    iterator test(): int {.closure.} =
+      while true:
+        block:
+          yield 12
+          hello()
+
+    let t = test
+    doAssert t() == 12
+
+  block:
+    proc hello(): int {.discardable.} = 12
+
+    iterator test(): int {.closure.} =
+      while true:
+        yield 12
+        hello()
+
+    let t = test
+    doAssert t() == 12
+