Fix type deduction for expressions when importing JSON

A recent fix for a separate issue
(#888) exposed a separate
bug: the type of expressions for which the top-level operator was
"access_field" was not deduced properly (set to UNKNOWN instead of
DATA), which caused bmv2 to assert when building the Action object.

The same issue existed for "access_union_header", which is an operator
introduced more recently (and possibly not used by p4c yet).

Fixes #889

src/bm_sim/P4Objects.cpp

@@ -198,6 +198,11 @@ P4Objects::build_expression(const Json::Value &json_expression,
     } else if (op == "access_field") {
       build_expression(json_left, expr);
       expr->push_back_access_field(json_right.asInt());
+      *expr_type = ExprType::DATA;
+    } else if (op == "access_union_header") {
+      build_expression(json_left, expr);
+      expr->push_back_access_field(json_right.asInt());
+      *expr_type = ExprType::HEADER;
     } else {
       // special handling for unary + and -, we set the left operand to 0
       if ((op == "+" || op == "-") && json_left.isNull())