Lex and parse support for ... statements and expand. (#7832)

No support for `each` yet, but this should be enough that we can try out
an implementation approach supporting expanding a statement across
elements of a tuple without building "real" variadics.

Assisted-by: Claude via Antigravity

toolchain/parse/handle_statement.cpp

@@ -51,6 +51,12 @@ auto HandleStatement(Context& context) -> void {
       context.PushState(StateKind::MatchIntroducer);
       break;
     }
+    case Lex::TokenKind::PeriodPeriodPeriod: {
+      context.PushState(StateKind::StatementPackExpansionFinish);
+      context.AddLeafNode(NodeKind::PackExpansionStart, context.Consume());
+      context.PushState(StateKind::Statement);
+      break;
+    }
 #define CARBON_PARSE_NODE_KIND(Name)
 #define CARBON_PARSE_NODE_KIND_TOKEN_MODIFIER(Name) case Lex::TokenKind::Name:
 #include "toolchain/parse/node_kind.def"
@@ -228,6 +234,13 @@ auto HandleStatementReturnFinish(Context& context) -> void {
   HandleStatementKeywordFinish(context, NodeKind::ReturnStatement);
 }
 
+auto HandleStatementPackExpansionFinish(Context& context) -> void {
+  auto state = context.PopState();
+
+  context.AddNode(NodeKind::PackExpansionStatement, state.token,
+                  state.has_error);
+}
+
 auto HandleStatementScopeLoop(Context& context) -> void {
   // This maintains the current state until we're at the end of the scope.