prevents declaring a constructor without importcpp fixes #22712 (#22715)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

tests/cpp/t22712.nim

@@ -0,0 +1,15 @@
+discard """
+targets: "cpp"
+errormsg: "constructor in an imported type needs importcpp pragma"
+line: 14
+"""
+{.emit: """/*TYPESECTION*/
+struct CppStruct {
+  CppStruct();
+};
+""".}
+
+type CppStruct {.importcpp.} = object
+
+proc makeCppStruct(): CppStruct {.constructor.} = 
+  discard