Golang: Skip ghost parameters in type parameter downcast (#5815)

### Description
Fixes #5814

### How has this been tested?
github-issue-5814.dfy

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

---------

Co-authored-by: Stefan Zetzsche <[email protected]>

Source/DafnyCore/Backends/GoLang/GoCodeGenerator.cs

@@ -1367,7 +1367,7 @@ private ConcreteSyntaxTree CreateSubroutine(string name, List<TypeArgumentInstan
           for (var i = 0; i < inParams.Count; i++) {
             var p = (overriddenInParams ?? inParams)[i];
             var instantiatedType = p.Type.Subst(thisContext.ParentFormalTypeParametersToActuals);
-            if (!instantiatedType.Equals(p.Type)) {
+            if (!p.IsGhost && !instantiatedType.Equals(p.Type)) {
               // var p instantiatedType = p.(instantiatedType)
               var pName = IdName(inParams[i]);
               DeclareLocalVar(pName, instantiatedType, p.tok, true, null, w);

Source/IntegrationTests/TestFiles/LitTests/LitTest/git-issues/github-issue-5814.dfy

@@ -0,0 +1,18 @@
+// RUN: %testDafnyForEachCompiler "%s"
+
+trait MyTrait<T> {
+  method Bar(ghost x: T, y: T) returns (z: T)
+}
+
+class MyClass extends MyTrait<int> {
+  constructor() {}
+  method Bar(ghost x: int, y: int) returns (z: int) {
+    return y;
+  }
+}
+
+method Main() {
+  var c := new MyClass();
+  var z := c.Bar(7, 42);
+  expect z == 42;
+}