bugfix: codegen issue that caused getGMTime() to leak memory

tests/gc/gcleak4.nim

@@ -6,7 +6,7 @@ when defined(GC_setMaxPause):
   GC_setMaxPause 2_000
 
 type
-  TExpr = object ## abstract base class for an expression
+  TExpr = object {.inheritable.} ## abstract base class for an expression
   PLiteral = ref TLiteral
   TLiteral = object of TExpr
     x: int

tests/gc/gcleak5.nim

@@ -0,0 +1,25 @@
+discard """
+  output: "success"
+"""
+
+import os, times
+
+proc main =
+  var i = 0
+  for ii in 0..50_000:
+    #while true:
+    var t = getTime()
+    var g = t.getGMTime()
+    #echo isOnStack(addr g)
+    
+    if i mod 100 == 0:
+      let om = getOccupiedMem()
+      echo "memory: ", om
+      if om > 100_000: quit "leak"
+     
+    inc(i)
+    sleep(1)
+
+  echo "success"
+
+main()