Fix type errors in function calls (#308)

* Make `field_list` always a list.

* Create separate `paren_expression` and `tuple` nonterminals

* Rename expression_or_field_list.* to field_list.*

Co-authored-by: Jeremy G. Siek <jsiek@indiana.edu>

executable_semantics/ast/field_list.cpp

@@ -0,0 +1,29 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "executable_semantics/ast/field_list.h"
+
+namespace Carbon {
+
+auto MakeFieldList(std::list<std::pair<std::string, Expression*>>* fields)
+    -> FieldList* {
+  auto e = new FieldList();
+  e->fields = fields;
+  return e;
+}
+
+auto MakeConsField(FieldList* e1, FieldList* e2) -> FieldList* {
+  auto fields = new std::list<std::pair<std::string, Expression*>>();
+  for (auto& field : *e1->fields) {
+    fields->push_back(field);
+  }
+  for (auto& field : *e2->fields) {
+    fields->push_back(field);
+  }
+  auto result = MakeFieldList(fields);
+  result->has_explicit_comma = true;
+  return result;
+}
+
+}  // namespace Carbon