hcl: RelTraversalForExpr must copy traversal before modifying it

The traversal returned from AbsTraversalForExpr may, for some expression
types, be referring to the same backing array as one stored inside the
node itself, and so previously this function may have inadvertently
corrupted the data associated with an AST node.

hcl/traversal_for_expr.go

@@ -52,11 +52,14 @@ func AbsTraversalForExpr(expr Expression) (Traversal, Diagnostics) {
 func RelTraversalForExpr(expr Expression) (Traversal, Diagnostics) {
 	traversal, diags := AbsTraversalForExpr(expr)
 	if len(traversal) > 0 {
+		ret := make(Traversal, len(traversal))
+		copy(ret, traversal)
 		root := traversal[0].(TraverseRoot)
-		traversal[0] = TraverseAttr{
+		ret[0] = TraverseAttr{
 			Name:     root.Name,
 			SrcRange: root.SrcRange,
 		}
+		return ret, diags
 	}
 	return traversal, diags
 }