fix sets of scoped imported enums (#13666)

tests/cpp/enum.hpp

@@ -0,0 +1,3 @@
+namespace namespaced {
+enum Enum { A, B, C };
+}

tests/cpp/tenum_set.nim

@@ -0,0 +1,9 @@
+discard """
+targets: "cpp"
+output: "{A, B, C}"
+"""
+
+type Enum {.importcpp: "namespaced::Enum", header: "enum.hpp".} = enum A, B, C
+
+var vals = {low(Enum) .. high(Enum)}
+echo vals