fixes #25803: add genCppConstructorExpr for full type-prefixed expression

tests/cpp/tcpp_default_ctor_assignment.nim

@@ -0,0 +1,14 @@
+discard """
+  cmd: "nim cpp $file"
+"""
+
+type
+  AmbiguousAssign {.importcpp, header: "tcpp_default_ctor_assignment.h".} = object
+    x: cint
+    y: cstring
+
+proc main =
+  var xs = newSeq[AmbiguousAssign](3)
+  doAssert xs.len == 3
+
+main()