Update executable semantics for choice to match final state of p0157 (#304)

* Make parentheses optional for alternatives with no arguments.

* Use `,` instead of `;` as separator for alternatives, and make the trailing separator optional.

executable_semantics/syntax.ypp

@@ -289,14 +289,26 @@ member_list:
     { $$ = $2; $$->push_front($1); }
 ;
 alternative:
-  identifier tuple ';'
+  identifier tuple
     { $$ = new std::pair<std::string, Carbon::Expression*>($1, $2); }
+| identifier
+    {
+      $$ = new std::pair<std::string, Carbon::Expression*>(
+          $1, Carbon::MakeTuple(
+            yylineno,
+            new std::vector<std::pair<std::string, Carbon::Expression*>>()));
+    }
 ;
 alternative_list:
   // Empty
     { $$ = new std::list<std::pair<std::string, Carbon::Expression*>>(); }
-| alternative alternative_list
-    { $$ = $2; $$->push_front(*$1); }
+| alternative
+    {
+      $$ = new std::list<std::pair<std::string, Carbon::Expression*>>();
+      $$->push_front(*$1);
+    }
+| alternative ',' alternative_list
+    { $$ = $3; $$->push_front(*$1); }
 ;
 declaration:
   function_definition

executable_semantics/testdata/choice1.6c

@@ -3,9 +3,9 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 choice Ints {
-  None();
-  One(Int);
-  Two(Int,Int);
+  None,
+  One(Int),
+  Two(Int,Int)
 }
 
 fn main() -> Int {
@@ -13,7 +13,7 @@ fn main() -> Int {
   var auto: y = Ints.One(42);
   var auto: n = 0;
   match (y) {
-    case Ints.None() =>
+    case Ints.None =>
       n = n + 2;
     case Ints.One(auto: x) =>
       n = x + 1 - 42;

executable_semantics/testdata/choice1.golden

@@ -9,7 +9,7 @@ var auto: x = Ints.None();
 var auto: y = Ints.One(0 = 42);
 var auto: n = 0;
 match (y) {
-case Ints.None() =>
+case Ints.None =>
 n = (n + 2);
 case Ints.One(0 = auto: x) =>
 n = ((x + 1) - 42);