Support for returned var and return var. (#3374)

Implement toolchain support for `returned var` and `return var`.

- Modeled `returned` in the parse tree as a `ReturnedSpecifier`
appearing after the `VariableIntroducer`.
- Modeled `return var` in the parse tree as a `ReturnVarSpecifier`
appearing after the `ReturnStatementStart`.
- Factored out the implementation of `return` statement and `returned
var` handling in check into a new `return.{h,cpp}`. The parse nodes
themselves are still handled in `handle_*.cpp`. This allows easy code
reuse between `return` and `returned var`.

toolchain/parse/handle_statement.cpp

@@ -38,8 +38,12 @@ auto HandleStatement(Context& context) -> void {
       context.PushState(State::StatementReturn);
       break;
     }
+    case Lex::TokenKind::Returned: {
+      context.PushState(State::VarAsReturned);
+      break;
+    }
     case Lex::TokenKind::Var: {
-      context.PushState(State::VarAsSemicolon);
+      context.PushState(State::VarAsDecl);
       break;
     }
     case Lex::TokenKind::While: {
@@ -179,8 +183,15 @@ auto HandleStatementReturn(Context& context) -> void {
   context.PushState(state);
 
   context.AddLeafNode(NodeKind::ReturnStatementStart, context.Consume());
-  if (!context.PositionIs(Lex::TokenKind::Semi)) {
+
+  if (auto var_token = context.ConsumeIf(Lex::TokenKind::Var)) {
+    // `return var;`
+    context.AddLeafNode(NodeKind::ReturnVarSpecifier, *var_token);
+  } else if (!context.PositionIs(Lex::TokenKind::Semi)) {
+    // `return <expression>;`
     context.PushState(State::Expr);
+  } else {
+    // `return;`
   }
 }