fixes implicitConv discarding flags (#24817)

follow up https://github.com/nim-lang/Nim/pull/24809
ref https://github.com/nim-lang/Nim/pull/24815

(cherry picked from commit 58b1f28177)

compiler/sigmatch.nim

@@ -2185,6 +2185,8 @@ proc implicitConv(kind: TNodeKind, f: PType, arg: PNode, m: TCandidate,
   # keep varness
   if arg.typ != nil and arg.typ.kind == tyVar:
     result.typ() = toVar(result.typ, tyVar, c.idgen)
+    # copy the tfVarIsPtr flag
+    result.typ.flags = arg.typ.flags
   else:
     result.typ() = result.typ.skipTypes({tyVar})
 

tests/tuples/ttuples_various.nim

@@ -1,4 +1,6 @@
 discard """
+targets: "c cpp"
+matrix: "--mm:refc; --mm:arc"
 output: '''
 it's nil
 @[1, 2, 3]