Fix codegen for set[T] parameters

Sometimes sets are materialized as arrays and we must treat them as
such: the CPP backend is pickier than the C one and would sometimes
produce invalid code.

Fixes #8967

tests/ccgbugs/t8967.nim

@@ -0,0 +1,10 @@
+discard """
+  targets: "c cpp"
+"""
+
+import marshal
+
+let orig: set[char] = {'A'..'Z'}
+let m = $$orig
+let old = to[set[char]](m)
+doAssert orig - old == {}