revert #12100; typeof(nil) is weird enough, don't allow it as the return type

compiler/types.nim

@@ -1310,7 +1310,7 @@ proc typeAllowedAux(marker: var IntSet, typ: PType, kind: TSymKind,
      tyNone, tyForward, tyFromExpr:
     result = t
   of tyNil:
-    if kind != skConst and kind != skParam and kind != skResult: result = t
+    if kind != skConst and kind != skParam: result = t
   of tyString, tyBool, tyChar, tyEnum, tyInt..tyUInt64, tyCString, tyPointer:
     result = nil
   of tyOrdinal:

tests/ccgbugs/tnil_type.nim

@@ -13,6 +13,3 @@ f3(typeof(nil))
 
 proc f4[T](_: T) = discard
 f4(nil)
-
-proc f5(): typeof(nil) = nil
-discard f5()