split the inline and closure iterators into different symbol kinds for easier discrimination between them

tests/exprs/texprstmt.nim

@@ -1,6 +1,6 @@
 discard """
   line: 10
-  errormsg: "value returned by statement has to be discarded"
+  errormsg: "value of type 'string' has to be discarded"
 """
 
 # bug #578

tests/iter/tanoniter1.nim

@@ -8,7 +8,7 @@ discard """
 """
 
 proc factory(a, b: int): iterator (): int =
-  iterator foo(): int =
+  iterator foo(): int {.closure.} =
     var x = a
     while x <= b:
       yield x