Fix bug in freshVarForClosureIter. Fixes #18474 (#19675) [backport]

* Fix bug in freshVarForClosureIter. Fixes #18474.

freshVarForClosureIter was returning non-fresh symbols sometimes.
Fixed by making addField return the generated PSym.

* remove discardable

Co-authored-by: Nick Smallbone <nick@smallbone.se>

tests/iter/tclosureiters.nim

@@ -21,6 +21,15 @@ discard """
 2
 70
 0
+(1, 1)
+(1, 2)
+(1, 3)
+(2, 1)
+(2, 2)
+(2, 3)
+(3, 1)
+(3, 2)
+(3, 3)
 '''
 """
 
@@ -152,3 +161,12 @@ var love = iterator: int {.closure.} =
 
 for i in love():
   echo i
+
+# bug #18474
+iterator pairs(): (int, int) {.closure.} =
+  for i in 1..3:
+    for j in 1..3:
+      yield (i, j)
+
+for pair in pairs():
+  echo pair