make tests green again

compiler/sempass2.nim

@@ -239,6 +239,7 @@ proc useVar(a: PEffects, n: PNode) =
         (tfHasGCedMem in s.typ.flags or s.typ.isGCedMem):
       #if warnGcUnsafe in gNotes: warnAboutGcUnsafe(n)
       markGcUnsafe(a, s)
+      markSideEffect(a, s)
     else:
       markSideEffect(a, s)
 

tests/effects/tgcsafe2.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: ''' (proc (s: string) = res &= &(s, "abc"), nil) is not GC safe'''
+  errormsg: '''type mismatch: got (proc (s: string){.locks: 0.})'''
   line: 11
 """
 #5620