Make choice work for alternatives without parameters (#4815)

This adds support for choice types at a similar level to that of a C
enum, where each alternative has a name but no additional
data/parameters attached to it. We generate a TODO diagnostic if
parameters are specified.

Because there's no extra data, the storage is a simple unsigned integer
discriminant of the smallest possible size.

A choice without any alternatives is not constructible. A choice with a
single alternative is, and has an empty tuple in place of a discriminant
since it has only one state. The empty tuple is used to make the class
non-constructible. This can be improved.

Each alternative is turned into a let binding on the choice that is a
value of the choice with that alternative set as the active one in the
discriminant. This isn't possible to write in user code with a class
right now, since the let binding has the same type as the choice
(which is a class) it is within. It's possible to generate it in semir
however by adding the binding after the class is marked complete.

toolchain/check/handle_binding_pattern.cpp

@@ -210,6 +210,19 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
           }
           break;
         }
+        case Lex::TokenKind::Choice:
+          if (context.scope_stack().PeekInstId().has_value()) {
+            // We are building a pattern for a choice alternative, not the
+            // choice type itself.
+
+            // Implicit param lists are prevented during parse.
+            CARBON_CHECK(context.full_pattern_stack().CurrentKind() !=
+                             FullPatternStack::Kind::ImplicitParamList,
+                         "choice alternative with implicit parameters");
+            // Don't fall through to the `Class` logic for choice alternatives.
+            break;
+          }
+          [[fallthrough]];
         case Lex::TokenKind::Class:
         case Lex::TokenKind::Impl:
         case Lex::TokenKind::Interface: {